The Underground World...
The underground world can be developed in different shapes and sizes and includes:
Adits are small tunnels for access into the undergroundout you.
Shafts are inclined or vertical excavations often circular in shape built to access deep locations underground
Tunnels are sub-horizontal excavations of sizes generally ranging from 2.5 m to 15 m
Caverns are built for underground storage or for the installation of equipment
Portals are the starting location and opening for a tunnel