The lab should have been hot. The deeper you go underground the hotter it gets and as deep as they were it should have been roasting (figuratively).
In the closing scene (when Walter and Florence are drawing on the blackboard) their electronic circuit additions are completely nonsensical and wrong.
Pressure is pressure and affects the body in the same way whether underwater or subterranean.
There is no lab anywhere near as deep underground as the one in the episode, but if there was the pressure would affect the team in the same way as being deep underwater, they would have had to slowly ascend to the surface with decompression stops to prevent the Bends, not in a rusty old mine lift (elevator).
Also, spending too long in the lab wouldn't have made them loopy without all the other damage caused by increased pressure.