Lincoln leads Clarke and Finn through the mines back to their camp in episode 1x12 "We Are Grounders: Part 1". Clarke and Anya have to escape through these tunnels in episode 2x03 "Reapercussions". Octavia and Indra lead and assault through the tunnels in episode 2x15 "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!".
From episode 1x12 "The Sound of Thunder, Part 1".