A magnitude 9.0 earthquake hit northern Japan early Friday, triggering tsunamis that caused widespread devastation and crippled a nuclear power plant. Are you in an affected area? Send an iReport. Read the full report on the quake's aftermath and check out our interactive explainer on Japan's damaged nuclear reactors.

[11:55 p.m. ET Wednesday, 12:55 p.m. Thursday in Tokyo] A Tokyo Electric Power company official said Thursday that - based on information gathered from a helicopter that flew over the Fukushima Daiichi nuclear power plant on Wednesday - authorities believe that there is water in a key fuel pool outside one of the plant's most troubled reactors.

"We have been able to confirm that there is water in the spent nuclear fuel pool," the official told reporters about the plant's No. 4 reactor. "But we do not know how much water."

Hours earlier, the head of the U.S. Nuclear Regulatory Commission testified that spent fuel rods in Unit 4 of Japan's stricken Fukushima Daiichi nuclear plant had been exposed, resulting in the emission of "extremely high" levels of radiation.

In addition, authorities announced the number of dead from the 9.0-magnitude earthquake and subsequent tsunami in Japan has risen to 5,178.

As of 10 a.m. Thursday (9 p.m. Wednesday ET), 8,606 people were missing and 2,285 were injured, the National Police Agency Emergency Disaster Headquarters said.

[11:05 p.m. ET Wednesday, 12:05 p.m. Thursday in Tokyo] Japanese Defense Minister Toshimi Kitazawa said that he and Prime Minister Naoto Kan decided early Thursday to commence the operation to drop water over the Fukushima Daiichi plant's No. 3 nuclear reactor. They also decided to send water-cannon trucks to the scene to spray water at the No. 3 reactor from the ground.

The plan to drop water from above had been aborted Wednesday, due to concerns about high radiation levels.

"We could not delay the mission any further, therefore we decided to execute it," Kitazawa told reporters.

[10:40 p.m. ET Wednesday, 11:40 a.m. Thursday in Tokyo] Japanese stocks rebounded slightly several hours into trading Thursday. After the Nikkei 225 index opened to a 397-point plunge, the drop was later cut to 204 points, a 2.3% decline.

[10:20 p.m. ET Wednesday, 11:20 a.m. Thursday in Tokyo] Helicopters carrying water made four passes over two nuclear reactors at the Fukushima Daiichi power plant in the first airborne attempts to address overheating. The helicopters, operated by Japan's Self-Defense Forces, have made four passes over the reactors in a span of about 15 minutes around 10 a.m. Thursday. Japanese public broadcaster NHK reported that water was initially dumped on the plant's No. 3 reactor, and then was dumped on the No. 4 reactor. Each helicopter was capable of carrying 7.5 tons of water.

[9:50 p.m. ET Wednesday, 10:50 a.m. Thursday in Tokyo] Nuclear engineers plan Thursday afternoon to begin restoring power to the stricken nuclear complex at Fukushima, a government official said.

"Today, we are trying to restore the power supply using the power lines from outside," said the official with the Nuclear and Industrial Safety Agency. "This is one of the high-priority issues that we have to address."

Once the power supply has been reestablished, then the cooling system will be operated using seawater, he said. He cautioned that the process will not be immediate.

[9:37 p.m. ET Wednesday, 10:37 a.m. Thursday in Tokyo] Water was dropped from helicopters Thursday morning over two nuclear reactors at the Fukushima Daiichi power plant, the first airborne attempts to address overheating inside related to emissions of radioactive material.

The helicopters, which were operated by Japan's Self-Defense Forces, made three passes over the reactors within 10 minutes before 10 a.m. Thursday. In the first two instances, water was dumped on the plant's No. 3 reactor, reported Japanese public broadcaster NHK, with the last being dumped over the No. 4 reactor.

[8:50 p.m. ET Wednesday, 9:50 a.m. Thursday in Tokyo] Temperatures recorded at spent fuel pools at the Fukushima plant Tuesday reached 84.0 degrees Celsius (183 Fahrenheit) at Unit 4; 60.4 degrees C (141 F) at Unit 5 and 58.5 degrees C (137 F) at Unit 6, the International Atomic Energy Agency said.

The agency said on Wednesday that "no data" registered for Unit 4, and Unit 5 had risen to 62.7 degrees C (145 F) and Unit 6 had risen to 60.0 degrees C (140 F). The temperature of these pools is normally kept below 25 degrees Celsius (77 degrees F)

Meanwhile, the United States is sending a radiation detecting aircraft to Japan. The WC-13W Constant Phoenix can detect radioactive clouds in real time, U.S. Air Force officials said.

[8:15 p.m. ET Wednesday, 9:15 a.m. Thursday in Tokyo]Stocks in Japan fell early Thursday morning there amid fears of a nuclear crisis following the nation's catastrophic natural disaster. The Nikkei 225 index, the most prominent measure of stocks traded in Tokyo, was down 397 points, or 4.4%, shortly after the market opened. The index recovered nearly 6% on Wednesday after plunging a combined 16% during the first two trading days following last week's massive earthquake and tsunami.

[7:01 p.m. ET Wednesday, 8:01 a.m. Thursday in Tokyo] As air carriers monitor radiation concerns at Japan's Fukushima Daiichi nuclear power plant, German airline Lufthansa and Italian carrier Alitalia have rerouted flights to and from the Tokyo area to other Japanese airports.

[5:54 p.m. ET Wednesday, 6:54 a.m. Thursday in Tokyo] More on the U.S. Nuclear Regulatory Commission chief's testimony on the "extremely high" radiation levels at Japan's Fukushima Diiachi nuclear plant: He said spent fuel rods in a spent-fuel pool in the plant's Unit 4 were exposed, and a hydrogen explosion resulted earlier this week.

Robert Alvarez, senior scholar at the Institute for Policy Studies, tells CNN that water had served to both cool the uranium fuel and shield it. But once the uranium fuel was no longer covered by water, zirconium cladding that encases the fuel rods heated, generating hydrogen.

That caught fire, resulting in a situation that is "very, very serious," Alvarez told CNN. He said the next solution may involve nuclear plant workers having to take heroic acts. Asked to be more specific, he said, "This is a situation where people may be called in to sacrifice their lives. ... It's very difficult for me to contemplate that but it's, it may have reached that point."

[4:25 p.m. ET Wednesday, 5:25 a.m. Thursday in Tokyo] A spent fuel pool at Japan's stricken Fukushima Daiichi nuclear plant is dry, resulting in "extremely high" radiation levels, the head of the U.S. Nuclear Regulatory Commission said Wednesday.

"What we believe at this time is that there has been a hydrogen explosion in this unit due to an uncovering of the fuel in the fuel pool," Gregory Jaczko told a House energy and commerce subcommittee hearing. "We believe that secondary containment has been destroyed and there is no water in the spent fuel pool, and we believe that radiation levels are extremely high, which could possibly impact the ability to take corrective measures."

[4:15 p.m. ET Wednesday, 5:15 a.m. Thursday in Tokyo] The Dow Jones industrial average ended the day's trading session 241 points lower amid concerns about the U.S. economy and the crisis in Japan.

[3:01 p.m. ET Wednesday, 4:01 a.m. Thursday in Tokyo] Concerned about potential radiation exposure, the U.S. military will not allow troops to get within 50 miles of the damaged Fukushima Daiichi nuclear plant, Col. David Lapan, a Pentagon spokesman, says.

Earlier, the U.S. Embassy in Tokyo said it was recommending that Americans who live within 50 miles of the plant evacuate or take shelter indoors. This is a break from the recommendation by the Japanese government, which has told people at least 20 miles away from the damaged reactors to evacuate.

Before Wednesday, the United States had urged American citizens in Japan to follow the recommendations of the Japanese government. The new U.S. recommendation "suggests that the advice the Japanese government is giving based on the information it has is different than the advice we'd be giving if this incident happened in the United States," White House spokesman Jay Carney told reporters.

[2:39 p.m. ET Wednesday, 3:39 a.m. Thursday in Tokyo] With less than 90 minutes left in the trading day, the Dow Jones industrial average is down 218 points as investors try to sort out not only disappointing U.S. housing data, but also concerns over developments in Japan.

[2:22 p.m. ET Wednesday, 3:22 a.m. Thursday in Tokyo] U.S. President Barack Obama was briefed Wednesday by the chairman of the Nuclear Regulatory Commission on the "deteriorating situation" of the damaged nuclear reactors in Japan, White House spokesman Jay Carney said.

[2:10 p.m. ET Wednesday, 3:10 a.m. Thursday in Tokyo] The yen reached a nearly 16-year high against the U.S. dollar in trading Wednesday, Japanese broadcaster NHK reported. Traders are speculating that Japanese companies will sell off assets to acquire yen that will be needed for rebuilding and recovery, the news agency said. At one point the yen was trading at 79.98 to the dollar, a figure not seen since April 1995.

[2:01 p.m. ET Wednesday, 3:01 a.m. Thursday in Tokyo] The U.S. Embassy in Tokyo is recommending as a precaution that Americans who live within 50 miles (80 kilometers) of the Fukushima Daiichi nuclear plant in Japan evacuate or take shelter indoors.

[1:26 p.m. ET Wednesday, 2:26 a.m. Thursday in Tokyo] The U.S. military will send an unmanned Global Hawk high-altitude reconnaissance plane to take photos and infrared images of Japan's damaged Fukushima Daiichi nuclear power plant, a Japanese government source told Kyodo News on Wednesday. The images could help workers figure out what's going on inside the radiation-contaminated buildings.

[12:44 p.m. ET Wednesday, 1:44 a.m. Thursday in Tokyo] International Atomic Energy Agency Director General Yukiya Amano will travel to Japan "as soon as possible, hopefully tomorrow (Thursday)," he said Wednesday, to get the latest on the situation surrounding Japan's nuclear plants and to see "how the IAEA can help them," he said. It will be a short stay of just one night, he added.

Also, U.S. military flight crew members given potassium iodide ahead of a mission in Japan showed no levels of radioactive contamination when they returned, military officials said Wednesday.

[12:33 p.m. ET Wednesday, 1:33 a.m. Thursday in Tokyo] U.S. Energy Secretary Steven Chu told a congressional committee Wednesday that the situation at Japanese nuclear power plants is "more serious than Three Mile Island," but Americans should rest assured that U.S. nuclear power plants are safe.

[12:09 p.m. ET Wednesday, 1:09 a.m. Thursday in Tokyo] The death toll in Japan stands at 4,314 as of midnight Wednesday, according to the National Police Agency Emergency Disaster Headquarters. At least 8,606 people were missing and 2,282 injured, officials said.

[11:45 a.m. ET Wednesday, 12:45 a.m. Thursday in Tokyo] The U.S. military is now giving potassium iodide pills to some helicopter pilots and crew members as a precautionary measure before missions flying into Japan, Pentagon spokesman Dave Lapan said Wednesday. Previously air crews had only received the pills after some missions.

[11:40 a.m. ET Wednesday, 12:40 a.m. Thursday in Tokyo] Japan's central bank made an additional 13.8 trillion yen ($170 billion) available to money markets Wednesday to cushion the country's banking system from the shock of Friday's earthquake and tsunami, Kyodo News reported. That brings the Bank of Japan's  emergency funding total to 55.6 trillion yen ($688 billion), Kyodo said.

[11:22 a.m. ET Wednesday, 12:18 a.m. Thursday in Tokyo] Ten aftershocks of magnitude 5.0 or greater were recorded Wednesday in the region of Friday's massive magnitude 9.0 earthquake in Japan, the U.S. Geological Survey reported. The greatest was magnitude 6.0, recorded shortly before 1 p.m. local time.

[11:18 a.m. ET Wednesday, 12:18 a.m. Thursday in Tokyo] The United States will measure radioactivity in Japan using its own equipment and 34 experts who arrived Tuesday, U.S. Ambassador John Roos told Japan's NHK news service. Roos said the U.S. seeks to help Japan as much as possible as well as ensure the safety of U.S. citizens there.

[10:17 a.m. ET Wednesday, 11:17 p.m. in Tokyo] The death toll in Japan stands at 4,277 as of Wednesday afternoon, according to the National Police Agency Emergency Disaster Headquarters.

[9:47 a.m. ET Wednesday, 10:47 p.m. in Tokyo] Delta Air Lines is pledging $1 million in cash and "in-kind support" to disaster relief efforts in Japan, the company announced Wednesday.

[9:27 a.m. ET Wednesday, 10:27 p.m.  in Tokyo] The National Police Agency reports 4,255 deaths from the earthquake and tsunami. An additional 8,194 people are missing and 2,282 were injured, the agency said. The number of dead is expected to go up as rescuers reach more hard-hit areas.

[9:15 a.m. ET Wednesday, 10:15 p.m.  in Tokyo] The governments in Iwate, Fukushima and Miyagi prefectures have asked the Japan Prefabricated Construction Suppliers & Manufacturers Association to build almost 33,000 homes to temporarily house those displaced by the quake and tsunami, Kyodo News service reports. At least 430,000 people are staying in shelters across eight prefectures, according to Kyodo.

[8:33 a.m. ET Wednesday, 9:33 p.m.  in Tokyo] Severe damage to the containment vessel of the No. 3 reactor at the Fukushima Daiichi nuclear plant is unlikely, Japan Chief Cabinet Secretary Yukio Edano said Wednesday, according to a Kyodo News service report.

[7:55 a.m. ET Wednesday, 8:55 p.m.  in Tokyo] Two U.S. military water trucks are being sent to help in cooling damaged reactors at the Fukushima Daiichi nuclear plant, the U.S. military says. The trucks will not be operated by U.S. military personnel, but by employees of Tokyo Electric Power Company, which operates the nuclear plant.

[7:44 a.m. ET Wednesday, 8:44 p.m.  in Tokyo] The number of dead and missing from Friday's earthquake and tsunami in Japan now exceeds 12,000 people, authorities said. The death toll stands at 4,164 as of Wednesday afternoon, according to the National Police Agency Emergency Disaster Headquarters. As of 6 p.m. (5 a.m. ET), at least 7,843 people were missing and 2,218 injured, officials said. The number of dead is expected to go up as rescuers reach more hard-hit areas.

[7:31 a.m. ET Wednesday, 8:31 p.m.  in Tokyo] Toyota Motor Corp. said Wednesday it will not resume vehicle production at its Japanese plants for another week. Parts production is expected to resume Thursday in an effort to make parts available for vehicles in Japan, according to a statement on the company's website. Parts production for overseas plants is expected to resume next Monday.

[7:06 a.m. ET Wednesday, 8:06 p.m.  in Tokyo] The number of nuclear workers at the damaged Fukushima Daiichi nuclear plant was slashed Tuesday from 800 to 50, but had grown to 180 by Wednesday afternoon, the Tokyo Electric Power Company said.

"Their situation is not great," said David Brenner, director of the Center for Radiological Research at Columbia University. "It's pretty clear that they will be getting very high doses of radiation. There's certainly the potential for lethal doses of radiation. They know it, and I think you have to call these people heroes."

[6:48 a.m. ET Wednesday, 7:48 p.m.  in Tokyo] Tests revealed traces of radiation in tap water in Fukushima city, 80 kilometers (50 miles) from the Daiichi nuclear plant, the local government said Wednesday. The Fukushima prefecture's nuclear department said amounts of radioactive cesium and iodine that are not harmful to the human body were found in water samples taken at 8 a.m. Wednesday (7 p.m. ET Tuesday). Government officials said the traces found are connected with the nuclear plant. A measurement of the tap water supply taken later in the day found no traces of iodine or cesium.

[6:40 a.m. ET Wednesday, 7:40 p.m.  in Tokyo] The earthquake and tsunami have left at least 1.6 million Japanese residences without water, public broadcaster NHK reports, citing Japan's Health Ministry. More than 300 water trucks have been sent to the hardest-hit areas, according to the report.

[6:14 a.m. ET Wednesday, 7:14 p.m.  in Tokyo] Continental Corporation, a German tire and auto parts maker, has flown its foreign staffers and their families out of Japan as a precaution. Its Japanese staff were being moved to locations in southern Japan.

[6:03 a.m. ET Wednesday, 7:03 p.m.  in Tokyo] The Korean Peninsula moved up to 5 centimeters (2 inches) to the east because of last week's quake off the coast of Japan, South Korea's Astronomy and Space Science Institute said Wednesday, according to the Yonhap News Agency. The quake moved Japan's main island of Honshu 8 feet (243 centimeters), the U.S. Geological Survey reported earlier.

[5:50 a.m. ET Wednesday, 6:50 p.m.  in Tokyo] A speech by Japan's emperor Wednesday marked the first time in Japan's history that an emperor has addressed his country during a crisis on television, according to the country's Imperial Household Agency. Emperor Akihito's direct appeal to the public - when he told citizens to not lose hope after last week's natural disasters - is also considered exceptional and extraordinary in Japan.

[5:41 a.m. ET Wednesday, 6:41 p.m.  in Tokyo] South Korea is sending more than 50 tons of boric acid, a material to help slow down nuclear fission reactions, to Japan for use Tokyo Electric Power's damaged nuclear facilities, Yonhap News Agency reported. South Korea's Ministry of Knowledge Economy said it was supplying the boric acid at the request of Japan after Tokyo used its reserves of the material at the damaged Fukushima Daiichi nuclear plant.

[5:22 a.m. ET Wednesday, 6:22 p.m.  in Tokyo] A Japan Self-Defense Force helicopter aborted its mission to drop water over the Fukushima Daiichi nuclear reactor because of radiation levels in the area, Japanese public broadcaster NHK reported.

[5:20 a.m. ET Wednesday, 6:20 p.m.  in Tokyo] Radiation levels taken Wednesday afternoon between 20 kilometers and 30 kilometers from the earthquake-damaged Fukushima Daiichi nuclear plant do not pose an immediate health risk, Japanese Chief Cabinet Secretary Yukio Edano said.

[5:15 a.m. ET Wednesday, 6:15 p.m.  in Tokyo] 80,000 Japan Self-Defense Force and police personnel have been mobilized for quake and tsunami relief, Kyodo News service reported Wednesday.

[4:53 a.m. ET Wednesday, 5:53 p.m. in Tokyo] The British Embassy will operate a coach from the disaster-ravaged city of Sendai to Tokyo on Thursday, the Foreign Office said.

A doctor in China said residents there need not panic about radiation from the damaged plant. "The winds are prevailing in a different direction, they're going eastward," said Dr. Philip Brooks at Beijing United Family Hospital.

[4:28 a.m. ET Wednesday, 5:28 p.m.  in Tokyo] The number of missing and injured people from Friday's quake and subsequent tsunami rose Wednesday afternoon, according to authorities.

As of 4 p.m. (3 a.m. ET), at least 8,181 people were missing and 2,218 were injured, according to the National Police Agency Emergency Disaster Headquarters. The death toll stood at 3,771.

[3:44 a.m. ET Wednesday, 4:44 p.m. in Tokyo] Japan Emperor Akihito told his nation to not give up hope and said the hearts of the international community were with Japan, in a rare, nationally televised address.

Emperor Akihito called Japan's earthquake and tsunami an unprecedented incident, saying the scope of the death toll is still unknown. However, the emperor's remarks focused on comforting a nation reeling from the disaster.

Emperor Akihito said he cares deeply for the Japanese people and was moved by his people's calm and order. He said in this kind of emergency, "We need to understand and help each other."

The emperor also said he was deeply grateful for the help from the international community and that the imperial household had received messages from all over the world that "their hearts are with us."

Akihito addressed the nation on national TV, a rare event that only occurs in times of war or national crisis.

[3:42 a.m. ET Wednesday, 4:42 p.m. in Tokyo] Japanese stocks rebounded Wednesday, with the leading stock index recovering nearly 6% from a two-day plunge stemming from the crisis created by the March 11 earthquake.

The Nikkei 225 index, the most prominent measure of Tokyo market stocks, ended up 489 points, or 5.7%.

The rebound came after intense selling over the previous two days, the first full-day sessions following the quake. On Tuesday, the index plunged 10.6%, marking the third worst one-day plunge in the Nikkei's history. The losses over two days totaled more than 16%.

[3:25 a.m. ET Wednesday, 4:25 p.m. Wednesday in Tokyo] Video from NHK Japan shows two helicopters in the air over reactor No. 3 at the Fukushima Daiichi. According to NHK, one helicopter is supposed to measure radiation levels at the reactor while the second helicopter is supposed to drop water on the reactor.

[3:07 a.m. ET Wednesday, 4:07 p.m. Wednesday in Tokyo] As a safety precaution, the Japanese government is now telling people living within a 10-kilometer radius of the Daini plant, the second nuclear power plant in Fukushima, to evacuate.

The Daini plant, which has four reactors, has reported fluctuating temperatures at its suppression pools. The Daini plant's reactors have been stopped, and there has been no radioactivity leakage so far.

[2:32 a.m. ET Wednesday, 3:32 p.m. Wednesday in Tokyo] French Prime Minister Francois Fillon suggested citizens who are not compelled to stay in Tokyo leave for France or the south of Japan now, according to an embassy statement Wednesday.

Fillon said the government has asked Air France to mobilize planes in Asia to respond without delay to requests for evacuations, and two aircraft were en route to Japan. Departures were scheduled for Thursday.

[2:20 a.m. ET Wednesday, 3:20 p.m. Wednesday in Tokyo] The death toll in Japan from Friday's 9.0-magnitude earthquake and subsequent tsunami rose to 3,771 Wednesday afternoon, authorities said.

[1:43 a.m. ET Wednesday, 2:43 p.m. Wednesday in Tokyo] Food safety officers at Indian ports and airports have been asked to test food that came to the country from Japan after March 11 for radiation, India's Health Ministry said.

[1:10 a.m. ET Wednesday, 2:10 p.m. Wednesday in Tokyo] Workers have returned to the damaged Fukushima Daiichi plant after an evacuation order was lifted, Tokyo Electric Power Company said.

[12:30 a.m. ET Wednesday, 1:30 p.m. Wednesday in Tokyo] The death toll in Japan from Friday's 9.0-magnitude earthquake and subsequent tsunami stood at 3,676 Wednesday afternoon, authorities said, though the number of missing people increased.

As of 12:30 p.m. (11:30 p.m. Tuesday ET), at least 7,843 people were missing and 2,044 injured, according to the National Police Agency Emergency Disaster Headquarters.

A white cloud of smoke or steam rising above Japan's Fukushima Daiichi nuclear plant may have been caused by a breach in the containment vessel in reactor No. 3, government officials said.

A spokesman for Japan's nuclear safety agency later told reporters that analysts were still trying to determine the cause of fluctuating radiation levels at the plant, but that radiation levels may have increased "because the containment vessel in reactor No. 3 has been damaged."

U.S. government experts trying to construct a model of radiation plumes emanating from the plant are being hampered by a "paucity of good data," a senior administration official said.

Workers at the plant have suspended their operations and been evacuated, said Chief Cabinet Secretary Yukio Edano said Wednesday.

Cold weather has increased the hardship for earthquake victims and rescuers. Conditions are expected to worsen, with temperatures forecast to drop below freezing by Wednesday across portions of the earthquake zone.

Stocks in Japan opened higher Wednesday morning, one day after the nation's main market index suffered one of its biggest drops on record. The Nikkei 225 index, the most prominent measure of Tokyo market stocks, rose 520 points, or 6%, shortly after the market opened.