@1a1nC Oceanic ones make more sense: much cheaper to get to for repair/maintenance, and it's a great heat sink.
It also has a lot of hard problems (I haven't worked on this, but I worked with people who had previously worked with the MS undersea datacenter).
Sea water is corrosive, so you need to have to have an internal cooling loop and do heat exchange, but piping sea water is annoying because it's entirely full of alive things. A lot of things in the sea have evolved to collect nutrients from moving water. Anything that looks like a narrow passage that water flows through is a perfect space for various families of seaweed and things like barnacles. And these things tend to be insulators. So then the heat is trapped in and the water heats up, then they die. And now you have a load of dead things in your cooling.
And a lot of these things grow really fast once they find a nice place to feed. Your cooling system looks just like their ideal underwater caves.
One of my favourite papers to review was from a Japanese team who had a material that you could wrap PCBs in and put them in water. One of their demos was to put a motherboard wrapped in their material in Tokyo bay in a plastic crate. When they got it out, the crate was completely full of seaweed. And that was without any active water flow.
