I created a huge Random map with an underground and 8 players and was happily expanding my empire when I suddenly realized I had explored all reachable areas in the underground region I started in and have run into solid rock on all possible borders. I am trapped in the bottom right 20%-25% or so of the underground map area, none of the sites I have found allow travel to the surface and after completing all my spell research, none of my spells allow me to send units to the surface either, so my game is toast.

You’d think the map generator would at least try and make sure any/all areas of the map are not isolated like this when it gets created. I now know why the underground region is turned off by default in the map generation settings, my guess is they are aware of this problem and failed to address it before the game went live.

That's unfortunately a glitch that's already reported on the Triumph forums. It will be fixed soon.
Only thing I can advise you to do right now is to start another map.

It is a rare issue, so getting twice in a row should be extremely rare (unless there's a specific combinations of settings that produces this result, in which case that would be good info for Triumph to have ^^).

damn that's gotta suck it's kind of a shame we can't build our own entrances to the surface but i understand why that is for gameplay purposes.