350 ft underwater: Maximum diving depth of a fully grown blue whale, the largest (known) animal on earth.
After 3,280 feet, light can no longer penetrate water.
5,280 feet is one mile.
13,120 feet underwater is called the Abyssal Zone. Water pressure around this level reaches around 11,000 pounds per square inch. Creatures such as the angler fish, the fangtooth, and the flashlight fish can survive at these depths.
20,000 feet underwater is called the Hadal Zone. Pressure levels reach around 16,000 psi, over 1,100 times the pressure on the surface. Although sparse, life exists at these levels.
31,000 feet is the average cruising altitude of the average commercial plane.
36,000 feet: the deepest known part of the ocean, located in Mariana's Trench. Only about ten percent of the ocean floor in this region has been mapped, so there are likely spots that are even deeper.
And that bit of knowledge gives me super heebie-jeebies.