She continues her journey downward, still going straight down. Now it is pitch black. No light, except possibly the bioluminescent glow of some of the deep sea creatures down here. How does she know which way is up? Echolocation? Or perhaps an unerring sense of gravity we don't have? Humans (and tortoises) would be hopelessly disorientated and confused.
After five and a half minutes the whale reaches five hundred meters (1640 feet). Humans can live and work down here but it takes days of compression to get there and days of decompression to get back. It also requires a mixture of special gasses. The sperm whale does it over and over again several times a day. The pressure is now over 700 psi (48 atmospheres).
After eleven minutes of steady swimming straight down, mostly in complete utter blackness, our whale reaches her happy hunting grounds. Now she is about 1000 meters (3280 feet or 3/5 of a mile) below the surface. Eleven football field lengths of water is above us. The pressure is 1421 pounds per square inch (almost 100 atmospheres). 200,000 pounds (100 tons) of water press on every square foot of the whale. All the time, day and night, winter and summer, the water temperature is 2 degrees celsius (36 degrees Farenheit).
This is the typical hunting depth for a sperm whale. Somewhere between 500 and 1000 meters. For the next 20 to 40 minutes our whale will stay down here in the dark and cold, hunting and eating. There are many unanswered mysteries about what goes on down here, but alas, we must save them for another article.
At this depth the air in the whale's body is one percent of its original volume, and it is 100 times more dense.
After spending 20 or 30 minutes catching and somehow swallowing a lot of fish and squid, our whale, still holding her breath, heads back for the surface.