If it got hot enough you could use a geothermal-type generator for reasonable efficiency, but that is (of course) still exploiting a temprature differential (a

If it got hot enough you could use a geothermal-type generator for reasonable efficiency, but that is (of course) still exploiting a temprature differential (a place where water remains liquid, and a hot place where it becomes high pressure steam to drive a turbine). However sadly that is unlikely to happen before the tunnels become unlivable :)