thumb|Area near the entrance of Raufarhólshellir showing Stalagmite#Ice stalagmites|ice stalagmites and part of the metal walkway thumb|Detail of the cave wall, showing bathtub rings and red-on-black coloration Raufarhólshellir () is the fourth-longest lava tube in Iceland. The cave's proximity to Reykjavík made it popular with visitors, who caused damage to the cave. In late 2016 the cave was closed to the public to clear accumulated garbage and install lighting and a walkway to part of the cave. The cave reopened for guided tours the following year.
thumb|Area near the entrance of Raufarhólshellir showing Stalagmite#Ice stalagmites|ice stalagmites and part of the metal walkway thumb|Detail of the cave wall, showing bathtub rings and red-on-black coloration Raufarhólshellir () is the fourth-longest lava tube in Iceland. The cave's proximity to Reykjavík made it popular with visitors, who caused damage to the cave. In late 2016 the cave was closed to the public to clear accumulated garbage and install lighting and a walkway to part of the cave. The cave reopened for guided tours the following year.
== Description == Raufarhólshellir is the fourth-longest lava tube in Iceland, at long, with a typical height of at least and width up to . The cave has multiple skylights, or holes in the ceiling, under which snow accumulates. Iceland Route 39 crosses over the cave at a point where it is about 15 meters in diameter. The cave hosts microbial mats containing a variety of microorganisms, including actinomycetota and acidobacteriota. The land containing the cave is owned by the Seventh-day Adventist Church and rented to a company that operates the guided tours.
Discovered by embedding cosine similarity (sentence-transformers MiniLM, 384-dim).