Nuclear fusion: Force two atoms together, hydrogen and indeed oxygen closer together to create fusion The immense pressure under the earth would suffice to force them close enough. It may ultimately be found that hydrogen and oxygen cannot exist very close together. The Hot ice shows that even at relatively low pressure the familiar water state is not observed.So what about all the other possible combinations of elements? There is a suggestion that there may be a lot of hydrides down there - to explain a possible discrepancy in the amount of Hydrogen expected on the surface.
But do you actually know the conditions needed for fusion to occur?
It needs more than 'it would be a nice idea'.