SOMA, Japan – Water levels dropped precipitously Monday inside a Japanese nuclear reactor, twice leaving the uranium fuel rods completely exposed and raising the threat of a meltdown, hours after a hydrogen explosion tore through the building housing a different reactor.

Water levels were restored after the first decrease, but the rods remained partially exposed late Monday night, increasing the risk of the spread of radiation and the potential for an eventual meltdown.

The cascading troubles in the Fukushima Dai-ichi plant compounded the immense challenges faced by the Tokyo government, already struggling to send relief to hundreds of thousands of people along the country's quake- and tsunami-ravaged coast where at least 10,000 people are believed to have died.

Later, a top Japanese official said the fuel rods in all three of the most troubled nuclear reactors appeared to be melting.

Of all these troubles, the drop in water levels at Unit 2 had officials the most worried.

"Units 1 and 3 are at least somewhat stabilized for the time being," said Nuclear and Industrial Agency official Ryohei Shiomi "Unit 2 now requires all our effort and attention."