Thermal http://www.youtube.com/watch?v=P-kGJ-mhfz0 (Inside Chernobyl's Sarcophagus)<br /><br />Caption: Elephant's foot. The molten core of Chernobyl (that part that didn't vaporize in the fire) flowed in a channel created by the structure of its reactor building and froze in place before a core-concrete interaction could happen. In the basement of the reactor at Chernobyl, a large "elephant's foot" of congealed core material was found. (Wikipedia - http://en.wikipedia.org/wiki/Nuclear_meltdown )
