When the Tok'ra arrive at a new planet, they immediately move beneath the surface and grow special crystals which bore through the ground, forming tunnels hidden from the Goa'uld. These tunnels do not typically have an entrance or exit shaft to the surface, but instead use Transportation rings. There may or may not be more than one transport ring system in a Tok'ra base. Life support is required but not immediately because as the crystals tunnel through certain materials, oxygen is produced as a by-product. This gives the Tok'ra time to set up air-scrubbers and other life support systems.