It's.....THE HOLE!!!
(version v5)
Units, features and projectiles that fall through the hole are removed from the game.
Picture
Minimap
springfiles link
a 14x14 map good for 1v1 to 4v4
It relies on luamex support (and includes its own gadget for it), which means that the metalmap shown on springlobby is blank, but you can see the spots easily on the minimap on the lobby clients.