SubTropolis

From Bloomberg:

The underground industrial park known as SubTroplis opened for business in 1964 in an excavated mine below Kansas City, Mo., attracting tenants with the lure of lower energy costs and cheap rents. The walls, carved out of 270-million-year-old limestone deposits, help keep humidity low and temperatures at a constant 68 degrees, eliminating the need for air conditioning or heating. Tenants have reported saving as much as 70 percent on their energy bills, says Ora Reynolds, president of SubTropolis landlord Hunt Midwest. Rents run about $2.25 per square foot, about half the going rate on the surface. "It's also a question of sustainability," says Joe Paris, vice president at Paris Brothers, a specialty foods packager that employs about 200 workers underground. In addition to Paris Brothers, 51 tenants have rented nearly 6 million square feet of space. Others include LightEdge Solutions, a cloud computing company that uses the mild climate to help cool servers, and an underground archive that contains the original film reels to Gone with the Wind and Wizard of Oz.
The U.S. Postal Service keeps hundreds of millions of postage stamps in an underground distribution hub at SubTropolis. There's still plenty of space here, with about 8 million square feet of land to develop—almost 10 times the floor area of Kansas City's tallest building. To reach capacity, Hunt Midwest may have to consider additional uses. Underground real estate has been used to grow mushrooms in Pennsylvania and vegetables in London. "We've talked about that," says Reynolds. "We've talked about fish, too. For now, we're trying to stick to what we're good at."

Makes energy sense.