What if a diver went deep down to the bottom of the ocean?

The maximum depth of a free diver is limited by how long he can hold his breath. The current world record is over 700 feet. It's a boundless record, achieved by riding a weighted sled and riding an inflated airbag. There are other disciplines with other records, such as constant weight, with / without fins, etc., they all have different records.

Currently, the world record for scuba diving is over 1000 feet. However, theoretically, a person could plunge unhurt deep into Challenger Deep (the deepest part of the ocean, in the Mariana Trench).

The main danger for diving too deep for too long is that the longer and deeper the diver is, the greater the accumulation of inert gases (nitrogen in the normal air, often helium in a deep dive), the more important it is. Accumulates in blood, these gases must be dissipated slowly and safely when the diver returns to normal atmospheric pressure.

If a diver climbs too quickly after a deep or long dive, the inert gases come out of saturation and the diver may feel the effects of decompression sickness or elbows.

On this Scuba Diving World Record, the diver came down in just 10 minutes, but it took him nearly 9 hours to get on and off slowly.

There is something known as saturation diving, where divers can live in a pressurized habitat for several weeks at depth, completely saturating their bodies with inert gas and never decompressing. Even if a diver is out of the water, he still breathes compressed air, as if diving. These types of diving habitats are beautifully featured in The Abyss, as well as in the Sphere book and movie. It's like being on a diving week. Saturation dives were conducted at depths greater than 1,700 feet and simulated in terrestrial hyperbaric pressure chambers at depths of approximately 2,200 feet. Decompressing such depths takes weeks.

To reach these depths, divers breathe a special gas mixture such as Oxygen, helium, nitrogen and hydrogen at different concentrations that change with depth.  Nitrogen can behave like a narcotic at high pressures and oxygen at high pressures becomes toxic. Thus, the nitrogen is replaced by helium and hydrogen, and the oxygen concentration is reduced, so that its partial pressure is below its toxic threshold.

Nevertheless, extremely deep divers experienced symptoms of the so-called high pressure nerve syndrome. Symptoms include tremors, memory loss, fatigue, etc. These effects induced by high pressures seem to define the limit of conventional air-supply diving.

Reference:

Deep Diving Maladies – High Pressure Nervous Syndrome

Deep Diving Maladies – Compression Arthralgia