Tunnels by which the Tok'ra conceal themselves underground. When Tok'ra arrive at a new base they immediately move beneath the surface, forming tunnels with special crystals. For the time being as the tunnels are formed, no life support is needed, as oxygen is produced as a byproduct of the construction -- if the crystals are used with the proper types of stone.
Different sizes and shapes of crystals produce different stretches of tunnel -- some long and straight, some leading upward or downward, some opening up rooms.
The tunnels are also capable of deconstructing themselves by command. The deconstruction process is particularly hazardous.
The Tok'ra, Part 1 - SG-1 first encounters the tunnels when they meet the Tok'ra.
The Tok'ra, Part 2 - The host of the Goa'uld spy Cordesh is killed in a collapsing tunnel. Once caught, the traitorous symbiote is condemned to the same fate.
Into the Fire - SG teams find a series of underground tunnels left by the Tok'ra, which help them rescue SG-1 from Hathor.
Last Stand - SG-1 uses crystal tunnels to escape from Anubis's forces on Revanna.
Fallout - SG-1 acquires crystals from the Tok'ra for their allies on Langara, helping Jonas Quinn to modify the D.U.E.V. to tunnel more efficiently.