Get inside the yellow doors, and mop the defenders up. You reach some stairs, winding up to the right, and behind them a column that drops fast and raise slowly.The column is your ticket to 3 secrets.

The easy one first: get on the column, and when it rises jump off to the left (seen from the stairs) into the toxic waste. At the wall, nearest the column, there is a small ledge with stuff.

For the next, ride the column up again, and jump off to the other side. Depending on your skill or selfconfidence, jump to the handrail or directly to the small platform. Do you feel lucky, punk?

This is not the actual secret. To get that, jump into the dirt (You ain't a Marine for nothing, right?) and run to the hazmat suit. Don't put it on yet. Opposite the suit, there is a button, that open a door - this is the secret. Don't forget to push the button inside, at the top of the stairs - it give you access to a chainsaw.

This is a bit of a no-brainer. Put on that suit and carry on to the next room. In the column right inside, there is a stash with some blue armour. Then carry on down the waste channel. When you pick up the Super Health shiny blue thingy, it triggers a door out and the secret. It is the only way out, so you better find it...

After you pick the blue key up, the room behind the yellow doors has been strangely crowded again. With your keen observer's skills you notice that a new room have opened between the yellow and blue doors.
When you tap the left side of the new room, a corridor to the outside - the secret - opens. Feel free to snipe away while you're out there.