Hanauma Bay is on the southeastern shore of Oahu and is an old volcanic crater that has opened on one side to the ocean. It is also the site of one of the most recent volcanic events that generated surface lava on the island of Oahu. I'm told that on the wave-cut bench that surrounds the bay on the right side (out of the picture), a darker-colored lava barrier is only about 10,000 years old. When you figure that Oahu has been around several million years, a mere 10K of that is like yesterday.
Hanauma is a place to look but not to touch. It has been a marine refuge for some time. The toughest part of diving there was getting a parking spot in the lot above. If you wanted to dive Hanauma, you needed to get there early.
From the beach, you can either swim out the channel through the shallow reefs and out into the central portion of the bay, or you can walk around the right side (looking seaward or “makai” as they say in Hawaii) out to a turbulent semi-cove called Witches' Brew, where you can do a giant step from the reef edge into several meters of water. (Just make sure you step far enough. One person in an Earthwatch project I was involved in didn't quite step far enough and sprained her ankle on the cliff edge just below the surface. It was too tough to climb back out with a sprained ankle, so we swam back to the beach--which was also tough, but easier than the climb.)
In the photo of the bay below, the shallow reefs at the bottom are just off the sandy beach. There is one channel, not visible in the photo, where it is easy to swim through the reefs. Witches' Brew is on the other side of the black point of rock that sticks out into the bay on the far wall of the bay a bit to the right of the middle of the shot.
My favorite Hanauma spot was over on the other side, although I'd usually get to it from the road above rather than from the beach. After passing the turnoff to Hanauma on Kalanianiole Highway, continue on a few hundred meters until the road started a sharp bend to the right. There used to be a wide area where you could pull off, although is now blocked by a road railing. We'd suit up here (where the first photo below was shot from) and walk a fair distance down the slope to a small narrow cove near the left side of the mouth of Hanauma. The cove was called the Toilet Bowl, named because swells would roll in and along the narrow cove, then push underground a short distance and bubble up in a rounded hole. Then it would drain back out like a toilet being flushed. The bowl was large enough for people to swim in, although I'm kind of surprised no one ever got sucked into the underwater cave and held down.
Rather than mess with the bowl, I'd usually enter the water near the mouth of the Toilet Bowl cove and swim out around the left side to the outside of Hanauma Bay. Out there are ledges, an interesting underwater archway, and a sheer cliff that has a couple of good-sized caves in it. It was always an interesting dive, although I have to say I never found all that many nudibranchs out there. The fauna is similar to the Kokohead dives just down the coast a bit. During the winter, you could often hear the songs of humpback whales from the outer cliff. They must have been passing by pretty close.
There is a shot of the Toilet Bowl cove. The flushing bowl is at the end over on the right. All the people you see hiked out along the edge of the bay from the beach.
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