Yea, you bet. They are cast concrete and steel and real stable/safe. The big one is the shaker house and as I remember is 4 stories. In footprint, they aren't huge so no matter where ever you are inside, you can always see daylight somewhere if a person is a little unfamiliar/uncomfortable with over-head environments. No danger at all, just fun. There are rooms, lots of steel stairs & railings, bunches of mechancal equipment, and A LOT of man-eating bluegill. The bottom floors of the shaker house and the bus are ~40-50' and you will want a hood and maybe some lite gloves. Most of the other stuff is ~20-30' and is easily bathing suit-able, however the bluegill are bad about not respecting personal privacy and are inclined to nip at one's ears & thighs.
Here is a link to their website:
http://www.marthasquarry.com/map.php
As I mentioned above, it's pretty thin and way out of date, but you can get the idea.
The page I am linking to has descriptions of some of the "stuff", but the photos in my OP are
way better than what's on the site.
There is plenty to explore and play with to last a good many tanks worth.
Have I got you sold yet?