What will happen if you get out of a submarine at the bottom of the Marianas Trench?

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1. To get out of the submarine at depth, you would need to open a hatch. The immense pressure would keep that hatch solidly closed against any force you might apply.
2. You still want to open the hatch so you need to equalize the pressure. You need to carefully open a valve to let water leak in. Just the barest little crack or the water stream will slice you in half like a laser through butter.
3. As the water slowly fills the sub, you’ll get quite cold, but don’t put on any type of sealed suit because as the water pressure builds, the suit will be compressed with you inside it.
4. As the water level rises in your submarine, it compresses the air pocket to the top. You’ll get to take some breaths as you shiver.
5. As the water continues to come in, the water pressure builds. Even if you brought along a SCUBA tank, you are going to lose consciousness due to extreme oxygen poisoning LONG before the pressure equalizes with the water outside. Once the internal water pressure approaches the equivalent of 500–600′ depth, you are out like a light…remember, your sub is sitting at 36,000 feet! Your brain has ceased operating to tell your lungs to breath. The sub still has some filling to do.
6. The sub continues to fill. An automatic hatch opener eventually senses the inside & outside of the sub are at the same pressure and opens the door. Your dead body drifts out.

The Trieste, a deep submersible submarine, went all the way to the bottom of the Marianas trench in 1960. On the way down, and close to the bottom, they heard a loud high-pitched ‘crack’ sound. When they got to the surface they found one of the porthole windows had cracked under the pressure but still managed to hold together. Otherwise the two scientists on board would have been instant paste as the water shot in.