Basically they,
when working as advertised by the accompanying experience, open up into a prior unmapped natural cave and their walls work identical (no clue what material the wall tiles are and no resource gained from hacking at it, while not uncovered by having mined an adjacent tile). While finding something in them would be nice (like some critters to study, eat or be eaten) I can live with the general idea, especially
because of them breaking the minesweeper routine.
Have you noticed that they seem to curve toward ore deposits?
Ceterum censeo
this defect of them has to go.