Among dive heads, Saipan is famous for this unique diving spot, a collapsed limestone cavern with a pool of cobalt-blue seawater filled by three underwater passageways. Sometimes the Grotto is calm and at other times powerful surges of water come whooshing in and out.

Once, locals who wanted to swim in the Grotto had to shimmy down a rope, but there are now steep concrete stairs down to the water. Tiny stalactites drip from above and massive spider webs hanging overhead make interesting photographs if caught in the right light. The glowing blue light at the bottom of the rock wall comes from the tunnels that lead to the open sea. There's a viewpoint looking down into the Grotto at the top of the stairs to the left.