Lincoln leads Clarke and Finn through the mines back to their camp in episode #112 "We Are Grounders, Part 1". Clarke and Anya have to escape through these tunnels in episode #203 "Reapercussions". Octavia and Indra lead and assault through the tunnels in episode #215 "Blood Must Have Blood, Part 1".
The Dominators set up in an old salt mine and the team heads there to rescue the President in episode #308 "Invasion!".