To begin with, pick up everything in the small room where you begin. Then go and guess the blue code at the panel on the wall - see Hints and Tips for more details. Then, exit the room and kill the two men on your left. Go up the stairs and kill the men there, then collect the ingram weapons - they will be very useful later. Next, walk back down to the start and drop into the water.
When you are in the center of the water, a lift will begin to push you up. Quickly jump off into one of the passageways, and collect everything you find, not forgetting to shoot anyone who opposes you. Repeat this for all the passages, then return to the beginning.
With your new found cache of weaponry, go up the stairs, and straight ahead, until you reach a second stairway. Climb these stairs, and at the junction, go left. Kill the man, and jump onto the platform overlooking some water. Collect all the ammo, and take out two pistols. Walk back to the junction in the stairs, but this time take the right passage. Mount the stairs, killing any opposition you meet on the way.
At the top, speed is of the essence, so kill people fast, but don't stand in one place for too long. Run across the wide area, and circle it killing people. Take the exit opposite the stairs, and select a powerful weapon. Walk into the corridor and destroy the sentry cannon (quite hard). Then fight your way to a large square room, and quickly run around the walls killing everyone.
Along one of the walls is a corridor leading up to a blank wall. On the left there will be a green switch, which you must press, and then run as fast as you can up the stairs. At the top, kill the small dinosaurs, and explore, staying in the orange area.
Now select the ingram weapon, and go out onto a bridge. You should come across a man with a plasma gun. Obliterate him ;-) then take his gun and continue through the orange area. Beware of a man with an rpg, the best way to kill him is to run towards him firing, dodging the rockets he fires. Work your way around the orange area and find the orange keypad.
Now go out onto the bridge area again, and work your way into the water (there's a couple of missiles to be found, if you look hard enough) and head downstream. You will come out (eventually) in a wide open space, but on the way down, a passage leading off is a good way of getting lots of ingram ammo.
At the bottom, select the plasma gun, and let rip against all the monsters. You should be able to wipe them all out, after a few tries, and go and collect the green code from one of the dinosaurs. Go in through the door, up the stairs, turn left and then go down the stairs.
Back at the main area, go and type in the orange keycode. The door opens, and you can swim down a lond way, then out into a large pool. Look around and find the missile launcher. Collect this and then head back out.
Out of the pool, head for a watery set of stairs. There will be a green keycode on the wall, so type in the code, and go up the steps. Walk into the capsule to complete the level.