The Tunnel of Chaos can be found after the What Lies Below quest. The entrance is east of Varrock, under the Saradomin statue that's just south of the bar west of the saw mill. You will need to bring a bronze pickaxe to excavate the statue.
It is also the safe shortcut to the Chaos altar for Runecrafting which avoids the Wilderness. The portal is at the far north-west end of the tunnel and requires a Chaos talisman or Chaos tiara in order to pass. The portal leads to the second level of the Chaos Maze. Head west to the ladder that you can climb down. Lastly, walk east to reach the altar.
If you stand facing the wall at the southeastern-most point of the lair, you can just make out the tournament field of the Duel Arena in the distance.
- Part of What Lies Below takes place in these caves.