When the Tok'ra arrive on a planet, they go deep underground and use technology to plant crystals which grow tunnels, similar to a geode. The network of tunnels creates an underground maze which can be structured to suit their purpose.
Individual crystals are specifically programmed to create different types of passageways. Small square crystals create short straight openings, long rectangles create longer openings, and diamonds tunnel up toward the surface. There is no need for ventilation shafts when growing the tunnels since the process uses rock such as calcium carbonate and produces oxygen as a byproduct, allowing enough time to enter the tunnels and set up the life support systems. When the Tok'ra abandon a planet, the process is reversed and the vanishing tunnels eliminate the network.
SG-1 has encountered Tok'ra tunnels on several planets including P34-353J, Vorash, Revanna, and Hathor's planet. When Kelowna faced annihilation by an impending explosion of naquadria, SG-1 cooperated with Jonas and Kianna to use a deep underground excavation vehicle to halt the advancing vein of the unstable mineral. Tok'ra tunnel crystals were used in conjunction with the vehicle's cutter heads in order to increase its speed and advance rate, and when the vehicle was unable to proceed, Kianna focused the energy of the crystals to create a tunnel big enough for her to climb the final kilometer to the target.
Cross Reference: Cordesh, Crystals, Kianna Cyr, Deep Underground Excavation Vehicle, P34-353J, Revanna, Tok'ra, Vorash
Episode Reference: The Tok'ra, Into the Fire, Crossroads, Divide and Conquer, Exodus, Summit, Last Stand, Fallout