Why do seashells make noise when I hold them up to my ear?

Shells hum because they catch quiet sounds. Their curls make those sounds louder. Like a cup over your ear. It is not the sea.

A seashell is like a little hard cup with a twisty space inside. Quiet sounds in the room and near your ear go in, bounce around, and come out louder, so you hear a whoosh. It sounds like the sea, but it is really nearby sounds, a bit like when you hold a cup to your ear and things sound boomy.

When you hold a seashell to your ear, it is not playing the ocean. The shell is a resonator, which means its hollow shape makes some sounds stronger. Tiny sounds around you, and even faint sounds from your body, go into the opening. They bounce off the curved walls, and the shell boosts some of them into a whoosh. Fun fact: different shells make different whooshes because their sizes and shapes boost different sounds.