The Ridden Hives in Back 4 Blood’s Tunnels of Terror expansion will challenge you and your team with constant Ridden threats while you explore their passageways for loot. It will be dangerous but should be well worth the effort if you survive. Among the loot is a secret golden skull trophy to collect. Here is where to find the one in 300 Below.
In 300 Below, your team needs to work their way down a few levels in a cave system to find the exit. The secret for this level is located on the bottom floor before you go towards the exit in the elevator shaft. You will have to drop down a couple of levels to reach it. Find the giant wheel in the screenshot below in the opposite direction of the exit.
When you find the wheel, go to the left side of the platform in front of it. Crouch and interact with the golden skull trophy between the platform and debris to claim it and earn the achievement or trophy associated with it.
Since the trophy is located on the same floor as the exit, you can return to this one if you pass it, but you may miss out on an Inner Lair entrance higher in the level.