Tunnels are found on planets all over the universe and may contain everything from precious minerals and gems to heavily shielded, disentigrator wielding robots.
Tunnels may have oxygen rich pockets in them that, if your away teams party is small enough, will refill your oxygen levels.
You may become trapped in tunnels if a liquid comes to block the way out. I.E. On a magma planet or water planet if water or magma flows onto the tunnel entrance, not only will your escape or entry be blocked but magma and water will start to flood into the tunnels becoming a very inevitable death trap.