Leap of Faith II

In research and development you'll go to the aquarium area.

Look down into where the water is pooling below and you should see hallway that you can jump into.

Here on the map:

Look down to see this:

Jump down and use your Air jets to propel yourself down there. Once you land in you'll get the achievement and if you keep running you'll see an elevator that will take you back up to the top level.