Every day for the past 20 years, Lyudmil Kutev has lumbered three stories down the crumbling concrete steps of his Sofia apartment, descended into a basement Cold War bunker packed floor to ceiling with shoes, and swung open a rusty window panel just inches from the sidewalk.

“My father hid here during the Allied bombing,” Mr. Kutev said, polishing a pair of loafers in the cramped space and peering up at the feet of those passing by outside. “And in the last years of Communism, he hid this shoe-repair business down here, too.”

Below Sofia’s Ottoman mosques, Red Army monuments, and onion-domed churches, some of the most intriguing relics of this city’s tangled past are lurking just below the sidewalk — and you’ll have to crouch down and peer through tiny windows to find them.

Known as klek, or squat shops, these knee-high ateliers and stores are nestled in former storage cellars and bomb shelters, and they’re only found in Bulgaria’s capital. Today, as a record number of tourists visit Sofia, these squat shops are emerging as some of the city’s most creative underground spaces.

Klek shop that has been closed for some time, with potential to become a creative space considering the art gallery upstairs (on the left), Hipster bar named after Spanish poet Lorca that can be hard to find (on the right)

“Visually, kleks are incredibly unusual, interesting spaces,” said Iara Boubnova, the director of Sofia’s Institute of Contemporary Art. “The fact that they force you to bend down and shift your perspective offers so much artistic potential.” In recent years, the institute has put on two pop-up exhibits inside kleks, and Ms. Boubnova is showing a new klek-inspired video installation through May 31 underneath Sofia’s Eagle Bridge.

Before World War II and during the Cold War, Bulgarians and Soviets designed bomb shelters in the basements of apartment buildings throughout Sofia, with separate spaces for each family. The rooms lining the perimeter often had a small window just above street level.

Huge store for household goods

As Communism unraveled in the late 1980s, many residents lacked money to open independent shops, so entrepreneurs started illicitly selling extra appliances and household goods while craftsmen offered shoe repair, tailoring, and other services to pedestrians from their dimly lit bunkers. By the 1990s, there were underground squat shops on most every street in Sofia.

Shoe repair shop located in the periphery of the Sofia center