This artificial cave will impress anyone who comes across it. A 14-foot-long tunnel opens up from the unsuspecting woodland, leading anyone who dares venture inside into a dark, igloo-like underground dome.

No one is quite sure who built the mysterious stone chamber. Because of this, the structure has been a point of curiosity for decades among locals—and a matter of debate among archaeologists.

Records show that that the land originally belonged to a local leather tanner, who many suspect built the chamber in the 1800s to use as a tannery. Others guess the builder could have been one of a number of farmers who lived around the land before that, who may have built the chamber to use as a root cellar or storage space.

The Upton Stone Chamber isn’t the only of its kind. It’s one of over 300 known mysterious stone chambers found across New England. Most archaeologists consider these chambers to be root cellars built by early European settlers, but others argue they’re the ruins of Native American structures.

You can find similarly beehive-shaped structures in other parts of the world, such as Ireland. This, coupled with the fact that the chamber appears to have been built in alignment with the summer solstice–a recurring theme in European megalithic structures—caused two archaeologists who studied the chamber in the 1970s to conclude that it’s proof Irish monks traveled to the Americas before any other known European expedition and built cave-like structure in the eighth century.

According to a 2015 U.S. Geological Survey, analysis of the chamber estimates its age as being built between 1350 and 1625, based on OSL Sampling. This has led some to conclude that it was built by the Nipmuc people, who lived in the area. The cave also lies along the Great Trail, a famed indigenous trade route that went from Delaware to Canada. Despite those findings, no Native American artifacts have been found inside the Upton Chamber.