Single-phase immersion

Submerge the entire board in a non-conductive fluid and the cooling problem largely disappears: no fans, no cold plates, no air path, and heat capacity orders of magnitude above air. It is rare at national-laboratory scale for reasons that have little to do with thermodynamics. Servicing a board means lifting it dripping out of a tank, no major vendor's warranty assumes it, and the fluid itself is a supply chain and an environmental question of its own. It is far more common in crypto-derived buildouts, where the hardware is uniform, disposable and nobody is maintaining a warranty.