Underwater Farting: The Physics and Biology of Intestinal Gas in Water
What happens when you fart underwater? The physics is clear: gas rises. But the behavior of intestinal gas in water is more complex than it seems — it depends on pressure, temperature, gas type, and depth. For scuba divers it's not a trivial question, for marine biologists a relevant one, and for everyone else: one that has occupied every human being at some point in the bathtub.
Buoyancy and Bubble Formation: The Physics of the Underwater Fart
Intestinal gas in water follows the laws of hydrodynamics. Since gas has a much lower density than water (air: ~1.2 kg/m³ vs. water: 1,000 kg/m³), a gas bubble in water experiences strong buoyancy and rises upward. The ascent speed depends on bubble size: small bubbles (< 1 mm diameter) rise slowly and can be deflected by currents. Larger bubbles rise faster and change shape as they do — initially spherical, then umbrella-shaped at larger diameters. Bursting at the water surface generates sound waves that humans perceive as a 'blub' — acoustically a brief, deep impulse. In the bathtub, this effect is particularly audible because the walls reflect sound waves, making the result seem louder than in open water.
Diving and Intestinal Gas: What Divers Need to Know
For recreational divers, intestinal gas is a practical though rarely discussed reality. With increasing depth, water pressure rises (by 1 bar every 33 feet / 10 meters), compressing the intestinal gas. At 33 feet (10 meters) depth, the gas has only half its surface volume. On ascent, it expands again — an effect divers notice. Medically relevant: divers who breathe compressed air (experiencing nitrogen narcosis at greater depths) should know that gastrointestinal problems under pressure can be perceived differently. The Valsalva maneuver — pressing against closed airways to equalize pressure in the middle ear — briefly increases abdominal pressure and can promote gas release. Among diving professionals, bubbles from the backside rather than the regulator are acoustically unobtrusive and tend to surprise dive buddies less than expected.
Marine Animals and Underwater Gas: Bubbles as Communication
Many marine animals use gas functionally. Fish have a swim bladder (gas sac) that regulates buoyancy — not intestinal gas, but also a biological gas volume. Herrings are known to produce high-frequency sounds through anal gas release (the phenomenon was scientifically described as 'Fast Repetitive Tick', FRT). Whether this is communication or a byproduct is debated among biologists. Sperm whale intestinal gas is considerable due to their size (up to 44,000 lbs / 20 tonnes) and their squid-rich diet. In pressure tanks for oceanographic research, gas collections from whale carcasses have been analyzed — the result is a cocktail of methane, CO₂, and various organic compounds. For divers near large whale groups, gas release from the animals is occasionally perceptible — as large-area bubble trails.
Why Bathtub Farts Sound and Smell Different
The bathtub is the everyday laboratory for underwater physics. A few observations with scientific grounding: Sound — An underwater fart sounds higher-pitched and more clipped than one in air, because water conducts sound 4 times faster than air and the bubble creates a brief surface impulse. The sound propagates simultaneously through water, bathtub, and air — a complex acoustic event. Smell — Hydrogen sulfide (H₂S), the main component of fart smell, is highly water-soluble. This means an underwater fart partly releases its odorous compounds into the bathwater rather than directly into the air. The air phase smells less intensely, but the water absorbs the odor molecules — which may explain why the bath briefly smells after an underwater fart, not the air. Bubble size — The 'Silent But Deadly' may reveal itself in water as many small bubbles rising individually.
Did you know?
- At 33 feet (10 meters) of water depth, intestinal gas has only half its surface volume — it expands back on ascent.
- Herrings demonstrably produce high-frequency sounds through anal gas release (scientifically: 'Fast Repetitive Tick', FRT).
- Hydrogen sulfide (H₂S) is highly water-soluble — an underwater fart releases odorous compounds into the water, not directly into the air.
- Underwater farts sound different from air farts because water conducts sound 4 times faster than air.
Fun Fact
The scientific discovery of herring communication via anal gas release (FRT) was awarded the 2004 Ig Nobel Prize for Biology. Researchers Ben Wilson and Robert Batty (Scotland) and Lawrence Dill (Canada) found that herrings release gas more frequently in the dark — possibly as a navigation aid. The joke within the research team: the abbreviation FRT was chosen deliberately. Their paper begins: 'We describe for the first time, to our knowledge, repetitive broadband sounds associated with the expulsion of gas from the anus of Pacific and Atlantic herring.'
Sources
- Wilson, B. et al. (2004). Pacific and Atlantic herring produce burst pulse sounds. Proceedings of the Royal Society B.
- Levitt, M.D. (1971). Volume and composition of human intestinal gas. New England Journal of Medicine.
- Nishi, R.Y. (1972). Decompression. Undersea and Hyperbaric Medical Society.