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. It is actually the hidden lair of the Dagon'hai. The area was released on 27 March 2007. You will need to bring a bronze pickaxe to excavate the statue. The toolbelt equivalent also works.
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, Chaos tiara, Omni-talisman, Omni-tiara, or Wicked Hood in order to pass.
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.