Raufarhólshellir Standard Tour, and the Ice at the Entrance

The ice is the first thing. Inside the entrance of Raufarhólshellir, water coming through the roof freezes into shapes that change with the season, and they are there before any of the lava formations are. Past them the tunnel runs on, one of the longest lava tubes in Iceland, and the guide explains what was happening inside the volcano when this was still a river of molten rock. This is the standard ticket, which covers the cave itself. Getting there is your own arrangement, and there is a separate version that includes transport from Reykjavík.