I am very very slowly working on my Deepest Sea Dungeon Master's Guide, but just to prove there is something going on, here is a d12 chart for "How old is that ruin that the PCs just found?"
I tried to go for a slightly archaeological look, with the PC party peering down through the dirt through the depths of time to the ruins. As you can see there is a better chance of "really old" ruins than of recent human ruins, which seems reasonable.
Thanks again as always to Telecanter for the silhouettes.
I asked potential players about a New Years game, and have had positive response, so some miniatures are on the way for that, and it gives me a good goal for getting the DM's Guide and a revised Player's Guide finished.