Our mountain is home to a wide range of geologic features, Aside from the wide range of volcanic features expected on a shield volcano, there are also glacial features and a small lake. Indeed, one could teach a fairly thorough course in geology simply visiting places on Mauna Kea. It is the lake that seems completely out of place, Lake Waiau should not exist.
The lake nestles in the crater of Puʻu Waiau, a low cinder cone on the south side of the summit at an elevation of 13,000ft (3,900m). It is not very large, about 100 yards across and about ten feet deep.
The lake is an anomaly. The cinder of Mauna Kea is highly permeable, water disappears into the ground at an impressive rate almost anywhere else on the summit. Something within the crater forms an impermeable layer that allows the water to persist at the surface.
The two most common theories are a layer of volcanic ash and/or clays, or a layer of permafrost beneath the surface. I prefer the ash and clay theory, there is suitable material present on several of the other cinder cones, most notably seen as yellow streaks down the side of Puʻu Poliʻahu and on the sides of Puʻu Waiau itself.
I have often been asked where the lake was, you can not see it from the road. There is no sign indicating the start of the trail to the lake. Not that the lake is any great secret, it is mentioned in all of the guidebooks and shown on the summit map distributed at the visitor center.
You can hike to the lake from above or below. You can start from the parking lot at 12,800 ft, just before Hou Kea Pass. Or you can start from the road intersection at the bottom of the summit bowl. Either trail skirts around the beautiful and symmetrical cinder cone of Puʻu Hou Kea to reach Puʻu Waiau. Both trails are noted on the map shown here.
Most folks use the upper trail to hike to the lake, myself included. The trail out to the lake is fairly easy, only a couple hundred feet of elevation loss to regain on the way back up. At over 13,000ft elevation you will feel every foot as you return to the road.
Lake Waiau is sometimes not the only body of water on the summit, though it is the only the permanent lake. Several other cinder cones will host lakes in their craters if there has been sufficient precipitation. I have seen an ephemeral pond formed after heavy rains in the crater of Puʻu Pōhaku, on the western side of the summit, that lasted about a week. I suspect that the same feature that preserves lake Waiau is at least somewhat present in this other crater.
Last summer the lake had shrunk to just a puddle in the middle, a rather sad state of affairs. After hurricane sourced rains it had recovered quite nicely, filling most of the usual area. When I last visited in the early fall of 2014 it looked fairly good. It has been a wet winter, with decent snowfall, I expect the lake is near its maximum level.
The lake is quite sacred to practitioners of the old Hawaiian beliefs. There are offerings and small ahu dotting the shoreline. Visiting the lake is a bit like visiting a temple, there is no doubt of the reverence this place holds.