The Rauðhólar ("red hills") are remnants of a cluster of pseudocraters in Elliðaárhraun lava fields on the south-eastern outskirts of Reykjavík, Iceland. The Rauðhólar pseudocraters are part of Reykjavík's nature reserve of Heiðmörk.
The age of the Rauðhólar is about 5200 years.
Originally there were over 80 craters, but the gravel from them was taken and used for construction. Most of the material was taken around World War II and used for projects such as Reykjavík Airport and road building.
Source: https://en.wikipedia.org/wiki/Rau%C3%B0h%C3%B3lar