In answer to how deep:
In 1960 Don Walsh and Jacques Piccard went to the bottom of the Marianas Trench (35,800 Ft).
That is pretty deep.
However in 2012 the Chinese reported
The submersible broke the 7000-meter barrier to reach a depth of 7020 . . . Before the Jiaolong was built, there were only four countries, the United States, Japan, France and Russia, that had deep manned submersibles. The maximum work depth of those countries’ deep submersibles is 6,500 meters; while the maximum designed work depth of the Jiaolong is 7,000 meters.