One of the challenges to CE is the inability to ‘tunnel’. Basically, the map uses terrain as a “surface” and if you break through that “surface” you “fall through the world.” That’s a problem for any tunneling mechanic.
But what about crafting tunnels “through” a mountain? This would be crafting a tunnel (say the size of a large gate) which is about three foundations wide. The crafted piece would only be one foundation wide. If the other end would not penetrate the other side (which one foundation wide probably never would) then the placement would render a brown, stone rock wall (much like the textures already in game). If enough of these tunnel pieces are stacked one after another so that the other side is breached, then the game would allow the light to stream through (and a gate could be placed on the other side – allowing people to move through the tunnel). If a piece gets destroyed, then add rock animation like a collapsing tunnel so that players don’t accidentally fall through the world.
Maybe add a ramp or stair to offset terrain height differences?
Hopefully, by allowing players to traverse through tunnels on built pieces would allow for a variant in base design and game play (being self-contained without having to actually build map layers for the removal of “dirt” in the existing map).
EDIT: This technique could only be used for exit from a base. Enemies would not be able to “tunnel” into a base because of the build restriction placement (we can’t build next to another player’s existing structure). So this would not be a viable base-invasion tool in PvP… but that doesn’t mean enemies can’t break down a gate and storm into a base through the tunnel. So exit points would still need to be guarded.