Every day, more than 150,000 people drive over the largest building in Kentucky and have no idea they did it. It's under the Watterson Expressway and about 70% of the Louisville Zoo, 75 to 100 feet down. Four million square feet, around 100 acres, seventeen miles of roads, and it stays 58 degrees all year.

It started as a limestone quarry. Miners blasted rock out from under this part of Louisville from the early 1930s into the early 1970s, leaving huge pillars of solid rock standing to hold the ceiling up. Then it sat as a dark, empty hole for about 17 years. In 1989 a couple of guys bought it. Most people saw an abandoned mine. They saw 4 million square feet of space a tornado can't touch, that never freezes or bakes, in the middle of a major city with highway access. Today it's high-security storage, zip lines, a bike park and a holiday light show. Louisville Metro and the state classify it as a building, and the ground on top and the space underneath even have separate zoning. A zoo on one floor and a warehouse district on the other.

That's the extreme version of something every land buyer should think about. When you buy real estate, you're buying a bundle of rights: the surface, what's underneath it, and the space above it. Those rights can be split up and sold separately. A few practical takeaways.

MINERAL RIGHTS

In eastern Kentucky this used to be a very big deal. During the coal boom, a lot of families signed what were called broad form deeds, which sold the mineral rights under their land while they kept the surface. Years later, coal companies used those deeds to strip-mine land the families still lived on. It got so bad that Kentucky voters passed a constitutional amendment in 1988 to limit them.

In Louisville, it almost never comes up on a normal house. But if you're buying acreage, farmland or commercial land, ask whether the mineral rights were ever split off from the surface. A good title company or real estate attorney checks this. It's one of the reasons title insurance exists.

KNOW WHAT'S UNDER THE HOUSE

More than half of the rock at the surface in Kentucky is limestone, and limestone slowly dissolves in water. That's how you get caves like Mammoth Cave, and it's also how you get sinkholes. Ask your inspector about drainage, foundation cracks and anything soft or sunken in the yard, especially on land and older homes.

LOOK FOR THE SPACE OTHER PEOPLE IGNORE

A dark, empty hole sat under Louisville for 17 years before somebody saw the opportunity. Most properties have a smaller version of that: the unfinished basement, the detached garage, the carriage house nobody uses, the extra half acre in the back. The value was already there. Somebody just had to see it.

A question for the group, because the underground angle got me thinking. Data centers are the most fought-over buildings in Kentucky right now. Oldham County ran off a $6 billion one planned on farmland and passed a moratorium, and one poll found 71% of people oppose a data center near them. The complaints sound the same everywhere: it's loud, it's a giant windowless box, and it eats farmland. Now picture one 100 feet underground. This is already being done. In Springfield, Missouri, there's one 85 feet down in an old limestone mine that stays 64 to 68 degrees every day.

Underground doesn't fix everything. It still needs a massive amount of electricity, and depending on how it's cooled it can still use a lot of water. But the noise, the eyesore and the farmland fight? A hole in the ground takes care of a lot of that. Heat seems to be an issue, so you'd have to figure out how to repurpose that.

Does anybody here own, or know of, old mines, caverns or quarries that could be repurposed? What would you do with 4 million square feet underground?