Is this related?
https://www.msn.com/en-us/weather/topstories/scientists-discover-massive-hidden-ocean-400-miles-beneath-earth-s-surface/ar-AA1F4Pqe#:~:text=A%20groundbreaking%20studypublished%20in,much%20water%20the%20planet%20holds.

I think that Reddit cephalopod anon said something similar.