Raudholar are the remainder of a cluster of pseudo craters within the boundaries of the capital. They date back approximately 4600 years and are situated in the so-called Ellidaar lava field, northeast of Lake Ellidavatn. They are prominent because of their reddish colour.
The scoria was coloured by the oxidation of the iron contents of the lava, when it overran bogs and small ponds on its way to the sea. The consequent steam explosions created the craters. There were about 80 of them before people started exploiting the scoria for different purposes.
During the years of the Second World War, most of the construction material mined there was used by the British occupation forces for the building of the runways of the Reykjavik Airport. In 1961 the rest of the craters was declared a national monument, and in 1972 the craters and a large area around them were proclaimed a nature reserve.