Glacierology 101: A moulin is a hole carved through a glacier by meltwater. Sometimes a moulin will grow until it becomes a cave in the ice, much like a blue slot canyon. And then, abruptly, the feeder stream will change course, allowing the cave to dry out and stabilize. Then it can be safe to venture into the cave and explore, which is exactly what I did on the Fox Glacier in New Zealand. Walking into the depths of the cave I watched as every color except blue was leached out of the light until eventually everything glowed with an internal cyan aura.