Secret Underground Lair

The Secret Underground Lair is found by going to The Falls in Tyria with M.O.X. in your party. It is the last stop in completing Zinn's Task.

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 (assuming you have completed the rest of the quest).

Overview
Dialog

Golem Sentry: *IN-TRUD-ER A-LERT*

Golem Sentry: *I-DEN-TI-FI-CA-TION VER-I-FIED, A-LARM DIS-A-BLED*

Zinn: ''Ah, you made it. Good. My intelligence network already informed me of the successful completion of your assignment.''

Zinn: ''I'm terribly relieved. I was horrified about what might have happened...to my reputation. Come along. Get your reward. You earned it.''

Zinn: Why, look who's here!

G.O.X.: *G.O.X. RE-PORT-ING FOR DU-TY*

Player: What does the "G" stand for?

G.O.X.: *I DO NOT KNOW*

Player: Is it supposed to be stupid?

Zinn: ''G.O.X. isn't stupid! It's very advanced!''

G.O.X.: *AD-VAN-CED*

Zinn: One day G.O.X. you will be just like M.O.X.  Isn't that right, G.O.X.?

G.O.X.: *JUST LIKE MON-GOOSE*

Zinn: No, no!  Just like M.O.X.

G.O.X: *JUST LIKE DOG*

Zinn: Oh, never mind.

Trivia

 * The G.O.X Dialog is a reference to the first Invader Zim episode where Zim receives Gir.