A pool brimming with hundreds of tonnes of spent nuclear fuel rods perched 30 metres above the ground in a shattered building next to a damaged reactor.
While much has been made of the meltdowns in reactors one, two and three at Fukushima, not so well known is the precarious state of the fuel storage pool in reactor four.
Japanese and US nuclear experts say if another large earthquake hits Fukushima and causes the damaged pool to drain, it could spark a catastrophic nuclear fire, which would release 10 times more radioactivity than at Chernobyl.
The operator TEPCO however, says the pool is stable and can withstand another large quake but admits it can't remove the rods immediately because vital equipment was damaged beyond repair.
Correspondent: Mark Willacy, ABC North Asian correspondent in Fukushima
Speakers:Robert Alvarez, former senior policy advisor to the US secretary of energy, Yoshimi Hitosugi, spokesperson for TEPCO, Hiroaki Koide, senior nuclear reactor engineer at Kyoto University, Mitsuhei Murata, Japanese diplomat
MARK WILLACY: They sit in the gloom, inside a pool open to the elements, five floors up, inside a shattered building.
1535 nuclear fuel assemblies, most of them containing highly radioactive spent fuel rods.
ROBERT ALVAREZ: The spent fuel pool in number four at Fukushima contains roughly ten times more caesium 137 than released by the Chernobyl accident.
MARK WILLACY: Robert Alvarez is a former senior policy advisor to the US secretary of energy. He fears another powerful earthquake could set off a catastrophe, bigger than what's already happened at Fukushima.
ROBERT ALVAREZ: The drainage of water caused by an earthquake or the toppling of the pool, which would be the worst possible consequence, could result in essentially the cladding around the spent fuel, which is made of an alloy of zirconium, to heat up and catch fire. And then there would be a massive release of radioactivity.
The major isotope of concern would be caesium 137. And it would involve about 10 times more radioactivity than released at Chernobyl.
MARK WILLACY: The operator of the nuclear plant, TEPCO, rejects this saying the fuel pool is stable.
But recently, the company was forced to admit that one of the reinforced walls of the pool had bulged slightly after last year's massive quake.
(Yoshimi Hitsogui speaking)
"We checked the condition of the wall the other day", TEPCO spokesman Yoshimi Hitosugi tells me. "The bulge was confirmed along part of it. But we do not think that it will have any effect on the soundness of the pool" he says.
But TEPCO's sanguine assessment of the fuel pool doesn't wash with some experts.
Hiroaki Koide is a senior nuclear reactor engineer at Japan's prestigious Kyoto University, and he warns that TEPCO is playing with fire.
(Hiroaki Koide speaking)
"If there's a crack in the pool and the water drains out the fuel rods will be exposed" Hiroaki Koide tells me. "It will then be impossible to cool the fuel. So if an accident happens, 10 times more caesium than has already been released by the Fukushima meltdowns will go into the atmosphere. Depending on which way the wind is blowing, Tokyo could become uninhabitable" he says.
MITSUHEI MURATA: I call it the sickness of Japan. Colloquially it can be explained that first, we hide; then we postpone; and then we assume no responsibility.
MARK WILLACY: Mitsuhei Murata is a former Japanese ambassador to Switzerland and a career diplomat who fears for the future of his nation.
Mr Murata is highly sceptical of TEPCO's assurances that everything at Fukushima is under control.
MITSUHEI MURATA: TEPCO and the government of Japan not only lacks the ability but the intention.
MARK WILLACY: So in your opinion, if there was a problem with that fuel pool it would be the end of Japan?
MITSUHEI MURATA: Yes. And there is no one who denies that.
MARK WILLACY: Until recently the only thing protecting the number four fuel pool from the elements was a plastic sheet.
Nuclear engineer Hiroaki Koide argues that TEPCO's main priority should be getting the hundreds of tonnes of spent nuclear fuel out of the pool and into safe storage.
(Hiroaki Koide speaking)
"As soon as possible, those fuel rods should be removed" he says. "Earthquakes are striking almost every day around the Fukushima plant, so I'm praying the big one won't hit" he tells me.
Despite Hiroaki Koide's anxiety, TEPCO doesn't seem to be in a hurry, and the company admits that even if it wanted to extract the fuel rods from the pool it couldn't.
(Yoshimi Hitsogui speaking)
"The original method was to take out the spent fuel by crane attached to the ceiling of the building", says TEPCO spokesman Yoshimi Hitosugi. "But that's been damaged. So we are thinking of installing a crane for this. We would like to start removing the fuel sometime next year" he says.
Until then, many in Japan are holding their breath.
And in far away Washington, former US government advisor, Robert Alvarez, fears a disaster greater in scope than Chernobyl.
ROBERT ALVAREZ: Nothing like this has ever happened before. This is a problem that, if such an event were to occur, it would be of an international dimension.
MARK WILLACY: So every time the earth rumbles, ripples spread across the surface of the number four fuel pool, and hearts flutter 240 kilometres away here in Tokyo.