In their ablation zones (where more snow and ice is lost from the surface each year than gained through snowfall), glaciers often have little stream networks running on the surface. In some places, those stream networks drain from the surface down to the bed of the glacier. They drain through things like this picture shows - moulins. This picture was taken from the glacier surface looking down into the moulin. This one is a few tens of cm across. Moulins can get huge though, and there were a few that we passed farther down the Llewellyn that were large enough to swallow a VW bus.