Gas is found practically everywhere: in the Earth’s atmosphere, deep underground in rock formations, dissolved in ocean sediments, inside your digestive tract, and even in your bloodstream. The answer depends on whether you’re asking about natural gas as an energy source, gases in the environment, or gas in the human body. Here’s where each type exists and why.
Gas in the Earth’s Atmosphere
The air around you is a mixture of gases, dominated by nitrogen at 78% and oxygen at nearly 21%. Argon makes up about 0.93%, and carbon dioxide sits at roughly 0.042%. Beyond those, trace gases like neon, helium, methane, and hydrogen exist in parts-per-million quantities. According to NOAA, these percentages describe completely dry air, but the atmosphere is almost never dry. Water vapor is nearly always present and can account for up to 4% of the total volume, varying by location and weather.
Natural Gas Underground
Natural gas, primarily methane, forms deep within the Earth over millions of years as organic material decomposes under heat and pressure. It collects in specific geological formations where the rock above acts as a seal, trapping the gas in place.
In conventional deposits, natural gas sits in porous sandstone or limestone where it can flow relatively freely toward a well. Unconventional formations are harder to access. Tight gas is locked in unusually impermeable sandstone or limestone that doesn’t let the gas move easily. Shale gas is embedded in dense clay-rich rock, making extraction more difficult and expensive. In both cases, the gas has been trapped for geological timescales, held in place by layers of non-porous rock overhead.
Coal beds are another source. Water and natural gas squeezed out of compressed clay migrate into more porous sand or silt layers nearby, where they accumulate. Each of these formation types requires different drilling techniques to release the gas.
Methane Beneath the Ocean Floor
Enormous quantities of methane exist frozen in ocean sediments as structures called methane hydrates, essentially methane molecules locked inside cages of ice. These form where water depths exceed 300 to 500 meters and temperatures are low enough to keep the ice stable. The hydrate layer can extend as deep as 1,000 meters below the seafloor before rising underground temperatures make the ice unstable.
Methane hydrates occur in seafloor sediments almost everywhere in the world’s oceans where depth conditions are right. They likely contain vast quantities of methane, though harvesting them commercially remains impractical. Interestingly, hydrates would technically be stable in the deep ocean water itself, but they don’t form there because the surrounding water isn’t saturated enough with methane to trigger crystallization.
Gas in Your Stomach and Esophagus
Your digestive system contains gas at every level, starting with the air you swallow. Every time you eat, drink, or even swallow saliva, a small amount of air enters your stomach. Most people don’t notice this, but monitoring studies show the average person produces hundreds of air swallows per day. People with a condition called aerophagia swallow air excessively, averaging over 500 air swallows in a 24-hour period, along with more than 120 belches as the body tries to release the buildup.
A second source of gas in the upper digestive tract comes from a chemical reaction in the duodenum, the first section of the small intestine. Your stomach releases acid, and your pancreas and intestinal lining release bicarbonate to neutralize it. That neutralization produces carbon dioxide directly inside the gut, adding to the gas already present from swallowed air.
Gas in the Large Intestine
The colon is where most digestive gas is produced. Trillions of bacteria ferment carbohydrates that your small intestine couldn’t fully absorb, generating hydrogen, carbon dioxide, and methane as byproducts. These three gases make up more than 99% of intestinal gas. The remaining fraction, less than 1%, consists of sulfur-containing compounds that are responsible for the odor despite being present in tiny amounts.
Not everyone produces the same gases in the same proportions. Your personal mix depends on which bacterial species dominate your gut. Some people harbor methane-producing microbes while others don’t, which is why breath tests measuring hydrogen, methane, and hydrogen sulfide are used to diagnose conditions like small intestinal bacterial overgrowth.
Where Gas Gets Trapped in the Gut
Gas doesn’t distribute evenly through your intestines. On abdominal imaging, the large bowel consistently holds the most gas, with the cecum (the widest segment, sitting in your lower right abdomen) being a common collection point. The ascending and descending portions of the colon are fixed in place along the sides of your abdomen, while the transverse colon and sigmoid colon hang on a flexible membrane and shift position.
Two sharp bends in the colon are particularly prone to trapping gas. The splenic flexure, located high on the left side where the transverse colon turns downward, is the highest-reaching segment of the colon. Gas rises to fill it, and when it becomes distended, it causes bloating, fullness, and left upper abdominal pain. This is common enough to have its own name: splenic flexure syndrome. A similar phenomenon occurs at the hepatic flexure on the right side, where the colon bends beneath the liver.
Gas Dissolved in Your Blood
Your bloodstream carries gases too, though you’d never feel them. Oxygen binds to hemoglobin in red blood cells, which increases the blood’s oxygen-carrying capacity by roughly 2,200% compared to oxygen simply dissolving in plasma. Carbon dioxide travels in the blood through multiple mechanisms: some dissolves directly in plasma (about 1.2 millimoles per liter at normal levels), but the total carbon dioxide content is 20 to 30 millimoles per liter because the blood has additional chemical pathways for transporting it, including conversion to bicarbonate and binding to proteins.
Nitrogen from the air you breathe also dissolves in your blood and tissues in small amounts. This is normally harmless, but it becomes relevant for scuba divers. Ascending too quickly allows dissolved nitrogen to form bubbles in the blood and tissues, causing decompression sickness.