A
clathrate is a mixture of water and a volatile gas, like methane, ammonia etc. They form ices where the water molecules in the ice trap molecules of the gases. The water molecules act like a cage, trapping the gas molecules inside a lattice of the water molecules.
You get a lot of methane clathrates on the ocean floor. Actually there's a significant amount of methane gas trapped in these clathrates and it's a potential fuel/energy source. It's also potentially hazardous to the climate if these things start to melt, and it doesn't take much to start the melting. They've had quite a few methane belches in the Gulf of Mexico over the years. Some have sunk ships. The most vulnerable clathrate deposits are those lying in shallow shelf seas (like the Gulf). If they sublimate, we could be in for some intense action later on so far as global warming is concerned. Methane is something like 25-40 times more effective a greenhouse gas than CO2.