[Fitzcarraldo]

burning "ice"

(methane hydrate)

[Phsstpok]

Methane hydrates are neat, but dangerous stuff. They are also pretty bulky for the amount of energy you get.

The undersea beds made up of hydrates also provide the most compelling scientific explanation for some of the Bermuda Triangle disappearances that I've even bumped into, though it does have some problems, as well.

http://www.geotimes.org/nov04/geophen.html