Wikipedia: "Kerið (occasionally Anglicized as Kerith or Kerid) is a volcanic crater lake located in the Grímsnes area in south Iceland, on the popular tourist route known as the Golden Circle. It is one of several crater lakes in the area, known as Iceland's Western Volcanic Zone, which includes the Reykjanes peninsula and the Langjökull Glacier, created as the land moved over a localized hotspot, but it is the one that has the most visually recognizable caldera still intact. The caldera, like the other volcanic rock in the area, is composed of a red (rather than black) volcanic rock. The caldera itself is approximately 55 m (180 ft) deep, 170 m (560 ft) wide, and 270 m (890 ft) across. Kerið’s caldera is one of the three most recognizable volcanic craters because at approximately 3,000 years old, it is only half the age of most of the surrounding volcanic features. The other two are Seyðishólar and Kerhóll.
While most of the crater is steep-walled with little vegetation, one wall is sloped more gently and blanketed with a deep moss, and can be descended fairly easily. The lake itself is fairly shallow (7–14 metres, depending on rainfall and other factors), but due to minerals from the soil, is an opaque and strikingly vivid aquamarine."