I grew up on a tiny island surrounded by a beautiful coral reef. Seeing the underworld through glass-bottom boats and snorkeling was greatly enhanced when the first underwater observatory was built. Basically, it was a structure built within a coral reef so you were standing surrounded by the sea life. Today, there is a wonderful aquarium that has sea water flowing in and out of it, is open at the top to allow the sun to impact the ecology of the sea world, and basically gives viewers as close to a natural experience as possible, given that it's an aquarium.
Being there has increased the pull within me to go scuba diving again . . . perhaps one of the most magical experiences in life.