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Why hasn't this taken up pace though? I am pretty sure I read about immersive cooling at least 4 -5 years ago. And 3M had some new "liquid" shown 1 - 2 years ago. Given the scale of which now Google, Amazon, Microsoft and Apple operate, surely they should be the first one to adopt?

Edit:

Looks like I asked the question a while ago. From Anandtech and Servethehome:

10 years ago, liquid had a thermal limit. Now it is around 5x that old limit. Problem is cost ($$$/gallon of the liquid) and installation. Has to be marketed on TCO. Also, issues with submerged fiber connections

2U air cooling can reliably handle 8x 200w+ TDP CPUs, 600w of NVMe, plus RAM and add-in cards. 4U designs can handle 6kW of cooling on air. Liquid may be more efficient, and more data centers are being built for it, but air is easy to deploy.