Why do seashells sound like the ocean when you hold them next to your ear?

Question: Why do seashells sound like the ocean when you hold them next to your ear?
- Keywords:
  ocean,
  seashell,
  sense,
  sound
Asked by gummybearnibbler to Emma, Jimmy, Janet, Niall, Simon on 18 Mar 2013.
- Niall Crawford answered on 18 Mar 2013:
  
  It’s actually the sounds of the blood running through your head.
- Emma Ashley answered on 20 Mar 2013:
  
  But it’s cool to think the seashell sounds like the sea, I love it! Another explanation is that the shape of seashells make them amplify the ambient noise around you.

Question: Why do seashells sound like the ocean when you hold them next to your ear?