How to get there: A signed cave at (-2846, 2067), in the meadow west of Hazelmere, past its hazel woods.
Eight centaurs (level 74) keep the halls, two in each long hall either side of a lit middle way. Each keeps to its own end of its hall, a pillar between them; the halls open on to the middle way in several places, so there is room to run and to kite. The middle way is clear of them but at the halls' mouths, and both ladders are well away from them.
At the north end of the upper halls a ladder goes down to The Mirrorwater, a round flooded hall where one hydra (level 92) waits by its black pool. Its heads grow back while it is left alone: strike hard and keep at it. Still pools and two pillars of rock give cover, and the ladder at the end of the southern passage climbs back up.