What does pent-up means

closely confined or held back.

Explanation:

If something's pent-up, it's restricted or held back in some way. You might appear calm and collected, but if you've secretly got a lot of pent-up anger, it will eventually need to be released. Boom! Use the adjective pent-up when you're talking about repressed emotions or stifled feelings or impulses.

"the sandy walls seemed to collapse in on themselves"

and

"the bloodshot waters seemed to have no bottom"