Paradise Glacier Ice Caves
Ice caves are really cool. I mean really cool. The air is just above freezing and they're wet from the melting glacier and stream flowing in the cave and out of the glacier. The Ice caves on Paradise Glacier on Mt. Rainier have been intermittent over the years, as noted in 1934 and 1987, and only reported intermittently since the late 1970's. I had the opportunity in the summer of 1977 to hike up from Paradise to see them, and like many people, I fell in love with Mt. Rainier NP. But it would be another 10 years before I had a chance to devote more time there.
No matter what you have seen in photographs of ice caves, unless the ice cave has a hole or a thin roof over it to let in more light, the latter meaning it's very unstable and may collapse, ice caves are dark very quickly hiking into them and produce a beautiful deep blue color to the walls, as seen the photograph. The NPS folks have an excellent introduction to glaciers and related features of glaciers such as ice caves.