These are the methane clathrates trapped mostly in the siberian sea (Arctic ocean), where increasing sea temperatures are causing methane to bubble up to the surface in large patches (sometimes 1 sq km in size).

Runaway global warming by melting methane clathrates is known as the Clathrate Gun Hypothesis: https://en.wikipedia.org/wiki/Clathrate_gun_hypothesis

In previous mass extinctions, such as the End of the Permian, 251 million years ago, 96% of species were lost - due to a similar event: " A cataclysmic eruption near Siberia blasted CO2 into the atmosphere. Methanogenic bacteria responded by belching out methane, a potent greenhouse gas. Global temperatures surged while oceans acidified and stagnated, belching poisonous hydrogen sulfide. "

https://cosmosmagazine.com/palaeontology/big-five-extinction...