Great discoverychannel.com article

Cassini flyby of Hyperion, one of Saturn's smaller moons, revealed an airless, tumbling "spongy" pile of ice, less dense than a snowball and possibly riddled with caves and other voids...

They were made public this week by scientists at NASA's Jet Propulsion Laboratory, along with a telling preliminary estimate of Hyperion's density: just 60 percent that of frozen water.
It's unlikely that a substantial portion of the moon is composed of anything less dense than water," said planetary scientist Jack Lissauer of NASA Ames Research Center. This strongly suggests that the missing 40 percent is nothing but empty space inside the moon, he said....

As for Hyperion's irregular shape, it's caused by its low mass and small size. It's not quite big enough for its own gravity to press materials together, fill its internal voids and round the moon into a globe. Put another way: "The internal strength is greater than the gravity," said Lissauer....

Interesting article I recommend reading it.

This was taken from 100,000 miles out at the closest. Hyperion is 175 x 100 km (117 x 67 miles) in size. Look at the size of those caves! Cassini will fly by at 317 miles on Sept 26. I'm Looking forward to close ups of those massive caves!

The voids they are talking about I guess could be likened to air bubbles in an ice cube? Could there be gases in the voids?

I remember discussing (not so seriously) a while back the possiblity of Amalthea being hollow. I know there are similarities in the low density readings. I've never seen a really good close up of Amalthea. They took no pictures on the flyby where they got the density readings.