Dive off the bridge. Go to the far-left corner. There is a hole in the roof, some armour inside.

In the room with the golden key is two slanted grids. The lefthand one, seen from the golden key, has some steps. The wall above have a walk-trough texture as door.

Watch yer ass when you grab the key.

Exiting from secret #2, jump in the small pool. It leads to another secret, goodies, and a slipgate.

Going trough the gold key door, to the right is a large lake. In the middle, on the bottom is a drain cover. Shoot it. Swim down when it opens. The corridor leads to a room with goodies, and a slipgate. Don't use the slipgate, before you collected secret #5...

...Go back down the submerged channel. Right under the drain cover you opened to get secret #4, is another drain cover in the side of the wall. Shoot it, and return to the slipgate room. Another secret room has opened there.

Get to the area, where a big steel cage encloses some computers. For your own good health, shoot the dogs before you open the door. Behind the first lift is a small room with ammo. Inside, on the left, is a lamp. Shoot it, a secret door opens.

Take the lifts to the top of the cage. Jump inside, and down to the bottom. You will find some armour there.