Question: What was the former function of 'The Mega Cavern' that passes beneath the State Zoo of Kentucky?

Context paragraphs:
Shymkent Zoo: The Shymkent Zoo (Kazakh: Шымкент хайуанханасы ; Russian: Шымкентский зоопарк ) is the state zoo of the city Shymkent in Kazakhstan.  It is one of the largest and oldest zoological parks in the country.  From the total zoo's area of 54 ha , under an exposition 34 ha .  The Shymkent Zoo contains more than 20 kinds of animals brought in the 'Red book'.  25% of requirements of forages for animals the zoo grow up on own earth, the area of 30 ha .

River Dulnain: The River Dulnain (Gaelic: "Tuilnean / Abhainn Tuilnein") is a major left bank tributary of the River Spey in northeast Scotland.  It rises in the eastern part of the Monadhliath Mountains ( ) and flows in a generally northeastward direction through uninhabited country to Sluggan, where it is crossed by Sluggan Bridge, constructed by General Wade to carry a military road.  (The route now forms a part of Route 7 of the National Cycle Network.)  Turning more easterly, the Dulnain passes beneath the modern A9 road which bypasses Carrbridge, the mainline railway and, in Carrbridge itself, the packhorse bridge that gives the village its name and the bridge carrying the B9153 road (former A9).  The final section flowing east-northeast to the village of Dulnain Bridge is accompanied by the A938 road.  The river empties into the Spey ( ) after passing beneath the A95 road and the disused bridge of the former Strathspey railway.

Ileosigmoid knot: An ileosigmoid knot is a form of volvulus in which ileum wraps around the base of the sigmoid and passes beneath itself forming a knot.

Louisville Zoo: Founded in 1969, the Louisville Zoo, or the Louisville Zoological Garden, is a 134 acre zoo in Louisville, Kentucky, situated in the city's Poplar Level neighborhood.  The "State Zoo of Kentucky" currently exhibits over 1,700 animals in naturalistic and mixed animal settings representing both geographical areas and biomes or habitats.

Alcatraz Dining Hall: Alcatraz Dining Hall, often referred to as the Mess Hall, is the dining hall of Alcatraz Federal Penitentiary where the prisoners and staff ate their meals.  It is a long wing on the west end of the Main Cellhouse of Alcatraz, situated in the center of the island.  It is connected to the block by a corridor known as “Times Square”, as it passes beneath a large clock approaching the entrance way to the dining hall.  This wing includes the dining hall and the kitchen beyond it.

Marsh Mills: Marsh Mills, also known as Haley's Mill or Spielman Mill, is a historic home located at Fairplay, Washington County, Maryland, United States.  It is a 2 ⁄ -story, three-bay-wide limestone house.  The structure was built about 1850 as a mill, then converted to a creamery in the 1880s.  Traces of the millrace are still visible where it exits the south end of the building and passes beneath the road.  Also on the property is an American Foursquare–style frame house which may incorporate remnants of the original log house which served as the original miller's dwelling.

Mega Cavern: The Mega Cavern is a former limestone mine in Louisville, Kentucky.  The cave stretches under parts of the Watterson Expressway and the Louisville Zoo.  Due to its support structures, it is classified as a building and is the largest building in Kentucky.  The cavern is used for business, storage, recycling, and tourism, with offerings including tram-guided tours, zipline tours, a ropes course, a mountain bike park, and an annual holiday lights display.

Assam State Zoo cum Botanical Garden: The Assam State Zoo cum Botanical Garden (popularly known as Guwahati Zoo) is the largest of its kind in the North East region and it is spread across 432 acre (175 hectare).  The zoo is located within the Hengrabari Reserved Forest at Guwahati, India.  The zoo is home to about 895 animals, birds and reptiles representing almost 113 species of animals and birds from around the world.

Butterley Reservoir: Butterley Reservoir is a reservoir in Derbyshire, England.  The reservoir was built to provide water for the Cromford Canal which opened for use in 1794.  The Codnor Park and Butterley Park reservoirs also provided water to the Cromford Canal.  The currently disused Cromford Canal passes beneath the reservoir along the Butterley Tunnel.  When the canal tunnel was in use water was provided to the canal directly from the reservoir via an adit which connects to the Butterley tunnel.

Wapping Tunnel: Wapping or Edge Hill Tunnel in Liverpool, England, was constructed to enable goods services to operate between Liverpool docks and Manchester, as part of the planned Liverpool and Manchester Railway.  It was the first tunnel in the world to be bored under a city.  The tunnel is 2030 m long, running downhill from the western end of the 262 m long Cavendish cutting at Edge Hill in the east of the city, to Park Lane Goods Station near Wapping Dock.  The Edge Hill portal is near the former Crown Street Station goods yard.  The tunnel passes beneath the Merseyrail Northern Line tunnel approximately a quarter of a mile south of Liverpool Central underground station.

Supporting paragraphs: Mega Cavern Louisville Zoo
Answer: limestone mine