The slope of the reef is such that if there is any sort of a groundswell, waves build up and break as they cross the stone.
RIBs have been flipped when they have got too close in a heavy swell, so take care.
Once under the surface, even in flat and calm conditions, there is a large surge on top of the Runnel Stone. Densely packed miniature plumose anemones in orange, cream, green and brown are everywhere. Huge ballan wrasse are busy selecting occasional morsels from the rock. A deep and smooth scour hole provides evidence of the raw energy of the sea. In 1923, the City of Westminster broke in two on the Runnel Stone rock in heavy fog and sank.