The majority of 'caverns' used for DUMB's were discovered and are millennia old structures with only the smaller ones and access structures being man-made.
It was mentioned that their size(empire state building roof height....) and numbers made flooding them all a non starter as it would noticeably lower sea levels....
and then there is this...
edit: trying to sniff out the root text but in any case it is from i_reddit_for_lulz and the relevant stuff is quoted(you also get more if you click the header text)
The majority of 'caverns' used for DUMB's were discovered and are millennia old structures with only the smaller ones and access structures being man-made. It was mentioned that their size(empire state building roof height....) and numbers made flooding them all a non starter as it would noticeably lower sea levels.... and then there is this...
https://archive.vn/jU2Qn
edit: trying to sniff out the root text but in any case it is from i_reddit_for_lulz and the relevant stuff is quoted(you also get more if you click the header text)