Sometimes, agates will have a little bubble of water trapped inside. This is called an enhydro. The water inside is millions of years old! The water can be seen if you shine a light on the agate from behind, or heard if you shake it. Eventually, the water will work its way out and evaporate via small capillaries, but the hollow part will still remain. It is also possible that water could enter the agate through the same capillaries. Enhydros can be found in watery places where agates are found, such as Brazil, Indonesia, and Oregon. Photos don’t show this very well, but on the link in the photo, the owner has some videos that show off the water inside.
For more information about enhydros, go here: http://www.rocksinmyheadtoo.com/Enhydros.htm