Víðgelmir Cave

The numbers are the point here. Víðgelmir runs 1,585 m through the Hallmundarhraun lava field, and at its widest the passage stands 15.8 m high and 16.5 m across. That makes it the largest lava tube in Iceland by a wide margin.

The lava that formed it erupted around 900 AD, so the cave is about as old as the settlement of Iceland, and people used it. Evidence of human habitation, probably from the Viking Age, was found inside and is now kept in the National Museum of Iceland.

The roof collapsed near the north end of the tube. The two openings it left are the only known way in. Tours follow a boardwalk several hundred metres inside, past lava formations and the colour bands left in the walls by the cooling flow, then return the same way.

• Dress for cold. The air inside is cold, and the walk from the warming station to the entrance is cold and windy. Warm layers, hat, gloves.
• Rings and small valuables go in a zipped pocket. Anything dropped on the lava floor is hard to retrieve.
• One low stretch. For a short section you duck; the rest of the route is open and walked on a boardwalk.
• Toilets at the site, and cards are accepted, including contactless.

Víðgelmir lies about 2 km southeast of the Fljótstunga farm in Borgarfjörður, West Iceland. A visit starts at Fljótstunga: access to the cave and the guided tours are arranged there, and the car park is at the site. The last stretch is on foot, from the warming station to the collapsed openings that form the cave entrance. It is out in the open and it is cold and windy, so put your jacket on before you set off rather than halfway.

Best: a wet or windy day. The tour is underground and goes ahead whatever the sky is doing, which makes Víðgelmir one of the few things in Borgarfjörður that bad weather does not spoil.

Avoid: the middle of the day, if you would rather walk with a small group. Later tours tend to be quieter.

Víðgelmir is a gated cave. An iron gate was fitted at the first constriction in 1991, to preserve the delicate lava formations that had not already been destroyed, and it still closes the cave to unaccompanied visitors. Beyond the gate you go with a guide, on a tour arranged at Fljótstunga.