Make your own way to the tunnel, just south-east of Reykjavik, or receive a pickup from your Reykjavik hotel. Wear sturdy shoes and be aware that to take the hike, you need to be in good physical shape as you may be walking underground for up to four hours.
On arrival, hear an introduction to the Raufarhólshellir, one of Iceland's longest lava ‘tubes.' Formed by lava streams from a massive volcanic eruption 5,200 years ago, the 4,500-foot (1,360-meter) tunnel is filled with lava formations and other natural monuments. Walking inside lets you view the inner forces of an eruption, and is especially memorable in winter when icicles and ice formations make the tunnel even more beautiful.
At the cave entrance, done the provided helmet, flashlight, and gloves with your small group of no more than eight participants. Then, follow your guide into the tunnel. View the columns of light shining down through the collapsed roof, and set off along the path, taking in the wondrous world around you.
Walk through the lava corridors, admiring the multi-colored rock layers and shapes as your guide explains the forces of nature behind them. Hike over the rough terrain, scrambling over rocks in places, and view the stunning ice pinnacles and formations created by cold air that gets trapped in the interior.
Toward the end of the tunnel, the Raufarholshellir splits into three smaller corridors. Hike to the end and see the magnificent lava falls and structures that have formed here.
Return to ground level after about four hours hiking underground.