Is it a canon canyon that was cored out by a cannon? Is that cannon even canon?
The canon cannons cannae cope, captain!
Folks are saying it's a quarry, and I'm inclined to agree. I'm no expert, but it looks very much like it was carved out artificially. Now, some might complain about it - "but there's no quarry in Iowa!!!" to which one can only respond that it's the future, and all those colossal buildings we see in the trailer must've come from somewhere.