You can go to a new sector of the world.
Choose world at the top right of the screen after you have logged into a world. This will give you a choice to relocate to a new sector. Choose a one, and all your bases will re-materialize roughly together and a few levels back from the base level you are in now. So if the forgotten bases and tunnels are around the 45 level, when you relocate expect to be in the 35-40 region of the new sector. Look for the alliances with the most active members - you can tell by looking at the alliance list if players have been logging in - and try to make contact with their players and get an invite.