There are a few (experimental) power station that get their power from the temperature gradient of the earth crust
There are more than "experimental" - In Nuclear Free New Zealand we've been seriously using the Wairakei Geothermal
power station since 1958, and we are currently increasing
the amount of geothermal electricity we generate.
It's a "sustainable" resource, but it's sadly not a renewable one.
Remembers... Hmmm Wairakei and Perl are linked for me... I first learned Perl in a tutorial at a Uniforum conference in the hotel at Wairakei (just around the corner from the powerstation) :o)