The Trieste was the first crewed Submarine to reach the deepest knowen spot on earth, the Challenger Deep in the Mariana Trench (10911m / 35797 ft) on 23 January 1960.
And this one can actually submerse and come to the surface again too. It does really work! Here is how it works:
-Throttle up and take her to the open sea(or better set a deep sea point)
-Start the Pumps with Act.1
-Than slowly VTOL up to submerse.
-Under water use Act.3 for forward movement
-VTOL in the middle to come up again. takes a while like the original.
Use roll to steer and Follow Cam.