To be honest, I think underground bases would add a huge benefit to the game. Why? Because it would give people something to do other than kill players. Everyone knows that the game has become a .50 cal death match with a few zombies as spectators, with a bowl of popcorn these days, however. I have been playing on Wastelands a lot and I seem to find that I spend more time building bases, finding gear to put in it and perfecting my layout more than I do going out and gunning down everybody in my path. It is more enjoyable in my eyes. Underground bases would be great, because then you can make trade havens, safe houses, possibly hostage rooms if you are a bandit. Tents wouldn't become obsolete in my mind, mainly because you will still have to gather all of the resources to build the base in the first place. So tents would be helpful storage containers for the materials. Plus, you could possibly use the tents inside the underground base to store things. These bunkers, or whatever you want to call them should definitely have a locking system, but they should also have a way of re-opening them if you are a hostile player to the bunker I.E. Satchel charge it to kingdom come. That'll be a fun way to enter a 'safe' house. However, I doubt you would be able to log out and back into the game in the base because you probably would spawn in the ground if you went onto a different server... Maybe I am wrong there, to be honest, you'll probably log in on top of where you were originally.