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Niagara Cave is a limestone cave approximately 5 miles (8 km) south and west of Harmony, Minnesota near the Minnesota/Iowa border in Fillmore County, Minnesota in the United States. An active underground stream contributes to one one of the cave's main attractions, one of Minnesota's largest waterfalls. Niagara Cave is a member of the National Caves Association and the International Show Caves Association.

Geology
Approximately 450 million years ago, during the Late Ordovician Period, southeast Minnesota was submerged under a shallow inland sea. The North American continent was oriented such that the equator ran from what is now the Gulf of Mexico (West) to the Hudson Bay (East), resulting in this region's climate being far warmer. These two factors contributed an abundance and diversity of life. Species of trilobites, brachiopods, gastropods, cephalopods, and more created their shells out of calcium carbonate (CaCO3) which would later result in the formation of limestone beds. Over hundreds of millions of years the limestone was covered by other sedimentary layers, the continents of the earth shifted to their current locations, and glacial activity worked and reworked the region north of Niagara Cave.

Formation
Slightly acidic water seeped into between natural horizontal and vertical cracks and began to slowly dissolve the crevices. The widening crevices allowed increasing amounts of water to enter until a fully developed stream was coursing through, eroding the limestone further. The majority of water that carved out the passageways was glacial water that flowed through southeast Minnesota as the glaciers from the north melted and receded. The water flowing through the developing passages also carried rocks and debris which helped carve the larger openings and produce uniquely shaped contours.

History
Niagara Cave was discovered in 1924.



About the Tour
Niagara Cave's namesake is a large waterfall which can be viewed only from above while standing on a bridge. This is a natural underground stream which may vary in volume as it depends on the amount of above-ground precipitation or snow melt. A great variety of fossils of organisms alive 450 million years ago may be seen on the tour as well. In the cave there is a wedding chapel which has hosted over 400 weddings since the cave was opened in 1934. The cave is a constant 48°F (~9°C) and is about one-mile round trip through canyon-like passageways with areas where the ceilings reach 100 feet (30 meters).