I've wondered whether little micro-filament heat pipes sandwiched right into the chips might be possible, all plumbed into a heat exchanger on the side of the die that can be directly cooled, facilitating the extraction of heat from deep within the stacked chips.
Maybe; there's https://www.tomshardware.com/news/tsmc-exploring-on-chip-sem... from about a year and a half ago.
Direct water cooling huh. With channels right in the silicon I imagine water purity is just that much more important.
God help you if the water gets gunked up with disintegrating gasket material[0] and blocks some of those channels off, or even worse, conductive stuff.
https://www.eetimes.com/ibm-git-demo-3d-die-with-microchanne...