Yards are generally aquatic invertebrate creatures indigenous to Maridia, a sector of subterranean lakes within the planet Zebes. They superficially resemble snails of Earth due to their eyestalks and large armored shell. In truth, this shell makes them extremely strong; the only known weapon capable of harming them is Samus' morph ball bombs. Despite these extremely durable shells, they generally hide within the various pipes that criss-cross Maridia. They can be startled back into their shells, and are harmless when inside of it; it appears to be a hobby of some more sapient organisms (including Samus, to an extent) enjoy kicking or juggling these shells. Once kicked back over, however, they will soon right themselves to continue on with their existence.