It’s drastically cooler underground until you get really deep in the mantle, and most importantly, the temperature is stable. No weather fluctuations.

Yes it is cooler in the beginning, but only for a short time, if you start pumping dozens (or these days hundreds) of megawatts of power into the ground it will heat up very quickly. Earth and rock is terrible at conducting heat.

Weather is not the issue for a datacenter, the heat generated from converting electrical energy, that’s the problem to solve.