The clathrate gun hypothesis suggests that the mass release of methane from methane clathrates on the ocean floor may have triggered catastrophic global warming, in turn causing mass extinction, at least once in the Earth's ancient past. Methane clathrates refer to methane gas trapped within water ice, discovered deep underneath ocean sediments worldwide. Methane clathrate is sometimes also referred to as methane hydrate or methane ice. The majority of it is thought to have been formed by microbes reducing (deoxidizing) carbon dioxide, converting it to methane.
Methane clathrates are not found all over the ocean floor — only on the continental shelves, the primary area of the ocean hospitable to life, and even there, found only in low concentrations, about 1% by volume. However, compressed in a cage of ice, the methane has a relatively high density. One liter of methane clathrate can contain about 168 liters of methane gas. Furthermore, methane gas is a greenhouse gas about 62 times more powerful than carbon dioxide. The clathrate gun hypothesis depends on the extremity of this warming effect.
The clathrate gun hypothesis begins with some external trigger, like the creation of a large igneous province or initial warming caused by sulfides released in a supervolcano eruption. The former is believed to have initiated the Permian-Triassic extinction, the greatest mass extinction in history, which killed off 99% of all species on Earth. As approximately a million cubic kilometers of lava was released over the course of a million years, from a massive volcano complex near the North Pole, huge amounts of lava crept out from the volcano and onto the continental shelves, melting the methane clathrates and releasing methane.
Although the methane only hangs around in the Earth's atmosphere for about 12 years, its release would have started a feedback effect, warming the Earth and making it more likely for further methane clathrates to melt. Under normal conditions, ice melts at 0 °C (32 °F), but the methane clathrates, some buried under more than a kilometer of ocean sediment, is under enough pressure to stay solid at temperatures up to 18 °C (64 °F). But if the temperature exceeds 18 °C, the methane clathrates are released — possibly in gigaton quantities. This would be devastating to all life on the planet.
The "gun" part of the clathrate gun hypothesis refers to both the fact that once it gets going, it can't be stopped, and its lethal effects. Once the planet starts warming, circulation in the oceans would decrease, causing large areas of ocean to turn anoxic, killing off life in huge numbers. Substantial data from the Permian-Triassic boundary has been found to synch well with the clathrate gun hypothesis, and now it is the foremost explanation for the cause of the Permian-Triassic mass extinction.