1. Chapter 3. As Soon As You Get Near The Gate Section, Go To The Right Side Of It
2. From There, Simply Go Down And Run To The End Of The Canyon
3. Enter The Tunnel
4. You'll Notice The Door With A Secret, But It's Closed
To open it, you should find a yellow machine within a red light and go behind it. There's a switch that opens the door.
Press it and go back to the door.
5. Enjoy!
I won't spoil what's behind this door but if you're a true Serious Sam fan you'll most likely enjoy it.
Source: https://steamcommunity.com/sharedfiles/filedetails/?id=2732230705
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