"Under" means something else is on top of it (e.g., water, earth, blankets, etc.). "Below" means something else is above it, but not on top of it (e.g., the second floor is below the third floor, the river bank is below the bridge, etc.). "Underneath" is similar to "under," but is used when describing a location (e.g., "The key is underneath the doormat").

the way they are spelled :P