Ok, so Brattygull1631 said there were some wired caves under his res, and showed us this. These are under the streets above, and were found by pure accident by digging down. These look kinda of like sewers and can be found at his res on SMP9. We are still not sure what the are. (any help admins?) We even found some mushrooms growing.
They were a result from the past residence owners who wanted that I guess. You can pay to make changes to the road, above and under.
I have the same thing, i hve 8 res's where the dirt under town around it is removed from the top to bedrock, i paid for it to be done, and i will be able to add stuff under the roads for my 8 res project,
Actually, I understand these holes to be related to World Edit. A very long time ago. Was not aware that it was copied over to the other servers also, like smp9
There was some original idea when emc first started to create sewers I think... but was never finished... And I believe SMP1 was copied to the other servers.