*Major Spoilers Ahead*
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Drink From The Water Cooler With The Head In It.
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This Can Only Be Done In Sahara Labs.
After You Kill The Boss And Find Timmy, Place The Boss In The Thing, Then Head On Over Towards The Second Artifact.
Equip The Rebreather And Go Through The Rock Cave In The Wall.
- After You Swim Your Way Through, Keep Going Until You See Labs Again.
The Water Jug You Are Looking For Is Located In A Lab/Office, On The Right Side Of The Hallway Here.
If You Get To The Elevator You Have Gone Too Far!
- Backtrack And Look To The Left (The Right On The Way In).
Drink From The Water Cooler With The Head In It And The Achievement Is Yours.
Gross!
Good Luck!