Players can enter an enemy base by swimming beneath the ground. To make this bug it is necessary to have two players, and they find the fault in some big stone that is on the ground. The fault appears to be related to this great stone. I don't know the exact procedure, but it takes two players and a big rock to enter the ground.
You must examine this circumstance. It also seems to me that these rogue bastards have already found another way to duplicate items.