There's a small peep hole about the size of a football directly above the well at ground level through which light passes. Apparently, when the moon shines over it at one point in its 18.6 year cycle it completely fills the hole. (Sound familiar, Callanishistas?) I managed to suppress my horrible small girl urge to spit through it and listen for the splash at the bottom.
To get down to the well you pass through a trapezoid-shaped hole and decend down into the ground on a stone staircase. The steps and the corbelling is so fresh and crisp that you feel it could have been built yesterday, though Julian in TME says that this is original stonework. If he's right, then this is truly astonishing. Likewise the beehivey conical corbelling leading up from the well to the peephole at ground level – incredible stonework. It really does look modern.
I descended down the crisp, steep staircase (suppressing further girlish urges, this time to kick out my feet and sing "New York, New York") about 3 metres below ground level to the water. Down there it was refreshingly cool; perhaps 10degC lower in temperature, but then it was 35degC outside.
Though I'm not big into wells, this one's a must-see.
A coachload of school children were at the well itself, so we wandered over to look at the hut beyond - a meeting hut due to the stone seat round the inside - and waited until they had gone to be able to fully appreciate the site.
An elliptical wall 26m x 20m, from the late Bronze Age, encloses the well temple. It's in 3 parts - a foyer, the steps, and then the well chamber itself.
The 24 steps are perfect. Smooth basalt, narrowing, creating a trapezoidal shape, echoed by the graduated ceiling, leading down to the well chamber.
The chamber itself has a bottle-like cross section, with the centre circular opening being about 50cm deep. The whole chamber is over 7m high and about 2.5m in diameter, again with perfectly smooth basalt blocks making a tholos style ceiling.
The water level was quite high, covering the step round the edge of the chamber.
Finds here have included a bronze ship figure, dated to 7th century BCE; the site is thought to be around 1000 BCE.