The top layer would be the surface level of the pool. Look beneath the top level. The next floor is full of rubble and has a big hole in the wall.
Photo available here:
The spent fuel pools are reported to have been 45 feet deep, according to the New York Times on March 15
[excerpt] "Mr. Lochbaum, who formerly taught reactor operation for the Nuclear Regulatory Commission, said the pools measured about 40 feet long, 40 feet wide and 45 feet deep. The spent fuel, he added, rested at the pool’s bottom and rose no higher than 15 feet from the bottom..."
Majia here: Interestingly, this article cites the fuel pool in reactor 2 as being damaged. We never hear anything about that unit:
[Excerpt] "A spokesman for the Japanese company that runs the stricken reactors said in an interview on Monday that the spent fuel at the Fukushima Daiichi and Daini plants had been left uncooled since shortly after the quake....
....The threat is considered so severe that at the start of the crisis Friday, immediately after the shattering earthquake, Fukushima plant officials focused their attention on a damaged storage pool for spent nuclear fuel at the No. 2 reactor at Daiichi, said a nuclear executive who requested anonymity because his company is not involved in the emergency response at the reactors and is wary of antagonizing other companies in the industry.
Majia here: One thing for sure is that there is no intact spent fuel pool left in 3 given what little is left of that building and the vast deepness of these pools.
Enviro Reporter Radioactive Nightmare