Limestone mining stopped at Mega Cavern in the 1970s, leaving around four million square feet of caverns on a single level extending under portions of Louisville (including the zoo). In 1989 it was purchased by developers to be turned into an underground business park. Today 700,000 square feet has been converted to this function, with around a dozen or so tenants, including Underground Vaults and Storage. A similar amount of space has been converted into a kind of underground amusement park, with a 350,000 square foot mountain biking course, zip lines, and aerial rope lines. A tourist tram runs regularly through the caverns, and seasonal lights and other displays are open to the public.