Hadal Trenches and the SOFAR Acoustic Channel

Mariana Hydrostatic Pressure, Trimethylamine N-Oxide Stabilizers, and Sound Waveguides

Named after Hades, the hadal zone comprises forty-six V-shaped trenches between 6,000 and 11,000 meters depth where oceanic plates plunge into the mantle. Sounded by HMS Challenger with hemp rope at 8,184 meters off Guam in 1875, the Mariana Trench's Challenger Deep reaches $10,925 \pm 12$ meters. That depth submerges Mount Everest beneath two kilometers of water.

To reach the Challenger Deep floor on January 23, 1960, Jacques Piccard and Don Walsh could not use an air-filled hull (which would implode at 1,086 atmospheres) or a nine-kilometer winch cable. Their 12.7-centimeter-thick steel sphere hung beneath the bathyscaphe Trieste's float filled with 85,000 liters of incompressible gasoline. To ascend, they dropped 16 tons of iron shot.

Although amphipods and worms thrive down to 10,925 meters, no fish has ever been filmed deeper than 8,336 meters. Paul Yancey proved in 2014 why vertebrates hit a wall at 8,400 meters. Bony fish stabilize proteins under pressure by adding trimethylamine N-oxide (TMAO) until at 8,400 meters internal TMAO hits the 1,000-milliosmole ceiling of seawater.

Described by Mackenzie Gerringer in 2017 as the deepest collected vertebrate down to 8,336 meters, the Mariana hadal snailfish (Pseudoliparis swirei) survives 800 atmospheres by abandoning bony armor. Because calcium phosphate bone turns brittle under extreme pressure, the snailfish deletes the osteocalcin gene. Its skeleton remains flexible cartilage and its skull roof stays open.

Scavenging at 10,900 meters in the Challenger Deep, the five-centimeter amphipod Hirondellea gigas faces zero fresh plankton and pressure that dissolves calcium carbonate shells. Hideki Kobayashi showed in 2012 and 2019 that it digests sunken wood into glucose using its own cellulase enzyme. It also extracts aluminum from clay to coat its shell in an aluminum hydroxide gel shield.

Paradoxically, mud at the 10,900-meter axis of the Mariana Trench consumes twice as much oxygen and holds triple the bacteria of the flatter 5,000-meter abyssal plain above its rim. Using autonomous landers in 2013, Ronnie Glud showed that steep V-shaped trench walls funnel earthquake mudslides. Those slides trap organic detritus in the bottom slot.

While seawater absorbs sunlight within 200 meters, it is a superb conductor of sound. Because water is 800 times denser and 15,000 times less compressible than air, its acoustic impedance ($Z = \rho c$) is 3,600 times higher than air's. Sound travels through the sea at 1,480 to 1,500 meters per second—four and a half times faster than in air.

Ocean sound speed is governed by two competing gradients: it slows down as water cools through the upper thermocline, and it speeds up as pressure squeezes water in the deep abyss. Maurice Ewing realized in 1944 that the minimum sound speed ($\sim 1,480\text{ m/s}$) at 1,000 meters depth acts as an acoustic lens. It refracts rays into the horizontal SOFAR waveguide.

Because sound inside the horizontal SOFAR waveguide spreads cylindrically ($1/r$) rather than spherically ($1/r^2$) and never hits the surface or bottom, a tiny pulse can cross a basin. When Maurice Ewing detonated a 4-pound TNT charge at 900 meters off the Bahamas in 1944, hydrophones recorded it 3,000 kilometers away. That test inspired a Navy pilot rescue system.

Long before submarines found the SOFAR channel, baleen whales evolved to broadcast into it. Roger Payne and Douglas Webb calculated in 1971 that the 188-decibel, 20-Hertz infrasonic calls of fin and blue whales lose less than 0.003 decibels per kilometer. Before ship propellers raised ambient noise tenfold, those 75-meter wavelengths crossed entire ocean basins.

To hunt giant squid at 1,500 meters in pitch blackness, the male sperm whale (Physeter macrocephalus) turns the front third of its body into an acoustic cannon. Forcing air past nasal phonic lips, the whale fires a click backward through two tons of spermaceti oil off a concave skull mirror. The focused 236-decibel pulse spots squid half a kilometer away.

Because sound speed in seawater rises 4.6 meters per second per degree Celsius, Walter Munk realized the SOFAR channel could act as a global thermometer. In the January 1991 Heard Island test, Munk's team lowered a 57-Hertz source into the polar SOFAR axis at 175 meters. Hydrophones recorded the pulse 3.5 hours later on both U.S. coasts 18,000 kilometers away.