Level 19: This Side Towards Enemy
As you arrive under water there's a gun lying on the ground behind you. From here you can either go forward or backward.

If you do decide to go backward you will eventually find a door blocking your path, if you take the route to the right. However, following the path to the left will lead to a room with a Save Terminal and two switches. The Marathon rule is to always activate any switch - do so.

If you went forward after arriving, you'll go through a door and find a tower. On the opposite side of the tower there's a platform. Ride this platform to the top level and jump to the switch on the opposite side. This switch will lower two platforms behind you.

As these platforms begin to lower jump to either.

If you follow the path you'll find a switch. Activating this switch will open the door that was blocking your path before. Now go back and jump to the other platform and repeat this task.

Now that the door is open you can get further up the tower. But, when you reach as far as you can, you will have to jump down to this room.

Once you have jumped down turn left and head up the stairs. At the top of the stairs is a room with a platform in it. If you stand on the platform it will lift you just enough to jump into the alcove in front of you.

This map details the location of that alcove. Study it carefully!

Once through to the room beyond watch out for the Troopers lobbing grenades. This room is similar to the previous room. In the center there's a platform and the rightmost pillar contains a pathway.

This is one of those fabled A.I. activation sites. Just hit the switch and activate it - you'll know when you've done it as it will light up. Once both sites have been activated process to the nearest terminal to get out of there.