Immersion cooling
Submerging whole servers in a non-conductive fluid. Sounds like a joke until rack density passes fifty kilowatts and air stops being sufficient.
There are two variants. In single-phase systems the fluid stays liquid and is pumped to a heat exchanger; it is simpler and uses relatively unremarkable oils. In two-phase systems the fluid has a boiling point just above the components' operating temperature, so it boils directly on the chip and condenses against a coil in the roof of the tank — heat transfer is dramatically better because the phase change carries the energy. The catch is that the fluids that work belong to the PFAS family, and 3M's announcement that it is exiting that manufacturing has upended the market's plans. The benefits are real: no fans, cooling overhead near the theoretical minimum, and recoverable waste heat. Servicing, on the other hand, is a wet business.