Discovered by workers in 1897 while digging a well for the winery 40 feet above, Crystal Cave is the world’s largest geode. The walls of this cave are covered in strontium sulfate, a blueish mineral called celestite. These crystals range from 8 to 18 inches long. The original cave was much smaller than what appears today, as crystals were harvested and sold for the manufacturing of fireworks.

Owned and operated by Heineman Winery, Crystal Cave helped save the family-run winery from demise during Prohibition. Tours of the cave helped pay the bills, while other wineries on the island closed.

Tours of the cave and Heineman Winery are offered for $8 for adults and $4 for children ages 6 to 11. Tours are offered seven days a week from early May until late September. Monday through Saturday, tours of the cave begin at 11 a.m. and continue throughout the day until 5 p.m. On Sunday, tours begin at 12 p.m. and continue until 5 p.m.