Methanobrevibacter smithii is a bacterium found in a dwarf's intestines. It prevents them from getting the "bends" when they accidentally tunnel into deep ocean. Mulch Diggums escaped from a prison shuttle in The Opal Deception by swallowing the air, causing decompression of the shuttle, which led to it breaking. Mulch then farted and swam to the surface, aided by the fact that he did not need to rise slowly.