Kerið looks at first like a quarry someone forgot to fill in, until you reach the rim. Then the whole thing opens below you: an oval crater with steep walls of rust-red and black volcanic gravel, streaked with green moss, and at the bottom a small lake of astonishing colour, deep blue-green and perfectly still on a calm day. It is a volcanic crater, formed a few thousand years ago when a cone of scoria collapsed into its own emptied magma chamber, and the lake is simply groundwater filling the bottom.
It sits beside Route 35, the Biskupstungnabraut, which runs north from Selfoss into the farmlands of the south and is the usual way into the Golden Circle from this side. The crater is on private land and the owners charge a small entrance fee, which pays for the paths. Allow half an hour to walk round the rim and down to the water, where you get a completely different sense of the scale, looking up at the coloured walls.
The country around is the broad, fertile lowland of southern Iceland, a plain of farms, horse pastures and rivers, with the hills of the Grímsnes volcanic field rising in low humps. This is some of the best farming land in Iceland, and the farms here have long histories, some going back to the settlement. Further north on the same road lie Skálholt, the old bishopric and centre of learning for centuries, and then Geysir and Gullfoss, the two big names of the circle.
As a ride, the Golden Circle is more about the stops than the road. Route 35 is a good, fast two-lane road with gentle curves and a few rises, and traffic is heavy in summer, with tour buses and rental cars, many driven by people who stop suddenly for photographs or horses. Wind is the factor to respect: it can come across the plain in sudden powerful gusts, and on a windy day it is tiring work. The light here is soft and changeable, and in the evening the colours of the crater are richest.
Selfoss, about thirteen kilometres south, is the regional town, with fuel, supermarkets, cafes and everything else. A curious footnote for chess fans: Bobby Fischer, the American world champion who spent his last years in Iceland, is buried in a churchyard just outside Selfoss. Hveragerði, to the west, is a town built on hot springs, where steam rises from gardens and greenhouses.