Gran Canaria (2017)
These holes in the rocks were carved thousands of years ago when the great Scandinavian Ice Sheet covered much of northern Europe. Many of the rocks in Norway are granites or gneisses, hard rocks to erode. On an ice sheet, occasionally water pools on top and breaks its way down to the base of the glacier, forming a deep hole called a moulin. As water spins its way down this vertical tube, it picks up sediment and carries it to the base, where it runs into the rocks at the bottom. As more water pours in, this sediment will abrade and erode the rocks at the base, wearing a roughly circular hole in the otherwise hard ground. Each of these ponds is several meters across. These kettles are found near Nissedal, Norway, but others are found around the world.
Image credit (CC licensed): https://flic.kr/p/WEMCPC