The pressures that they were working on are absolutely incredible.
There are wells under much higher pressure ...on land...in Oklahoma in the Anadarko Basin.
Measured pressures inside the reservoir were between 10 and 13,000 psi. This may sound "absolutely incredible", but I can assure you that it is not. The chemical mfg. that I once worked for had several processes that ran at 20,000 psi 24/7/365.
The 100,000 psi number that keeps getting tossed around is garbage.