Secret Underground Lair

The Secret Underground Lair is found by going to The Falls in Tyria with M.O.X. in your party.

Getting there
Start from Ventari's Refuge and travel south through Reed Bog to The Falls. From there, follow the path on the map to the right until you come to the collector Agrippa Stonehands. Behind him will be an interactive object labeled "The Falls"; click on it to enter the Secret Underground Lair.

Once you are in the Secret Underground Lair, there are no foes. Just go see Zinn for your rewards.

Trivia
The Secret Underground Lair has the same layout as level 2 of Oola's Lab.