Caves that are filled with water very rarely collapse. The pressure of the water inside helps support the roof, so in that respect, they are safer than dry caves. Of course there is that little detail of no air to breathe.. The presence of all the dripstone formations, stalactites and stalagmites etc, shows that this cave was dry for a long time when the sea level was lower, after the cave itself was formed. While I'm no expert,it looks like is a solution cave like most of them - rain water picking up acid from the air and soil dissolves the limestone layers where it flows underwater. It takes a thousand years for the spaces to become big enough for a human to pass! So the big cathedral-like rooms of Giant Cave just boggle my mind when I try to think how old they may be.