IN HOUSTON’S DOWNTOWN tunnels on a recent morning, John Czach planned to cover approximately five miles with a group of urban hikers before the lunch crowd descended.

The pace would be quick as the group explored the nooks and crannies that would reveal interesting bits of Houston history.

As organizer of a MeetUp group called Houston Urban Trekkers, Czach, 61, typically leads hikes through the forested trails of Memorial Park or the scenic areas of Tanglewood. But a few years ago, after his co-organizer suggested he plan a route through the tunnels, Czach created a hybrid hiking/history tour that never fails to fascinate transplants and native Houstonians alike.

“There’s cool stuff down there,” said Czach, a volunteer at the Houston Museum of Natural Science and native Houstonian history buff, with an eclectic collection of local maps, newspapers, and political ads to prove it. “I tell people that I do hikes in the tunnels, and some don’t know the tunnels exist.”

Built in the 1960s to link buildings and garages and shield downtown workers from Houston’s brutal heat, today the tunnels run roughly seven miles, connecting 95 city blocks, 24 buildings, and countless restaurants and shops. Czach views them as enclosed trails, and he isn’t wrong: They’re perfectly conducive to the brisk, 18-minute miles he likes to maintain as he leads groups of roughly six to 12 people—some regulars, some simply curious—on his expeditions. The underground hikes, offered several times a year, are free.

“John knows so much about Houston,” said Suzanne Risley, a fellow hiker and high school history teacher who regularly goes out with the Urban Trekkers. “Whenever he’s leading a hike, you know you’re going to learn something.”

Czach’s routes incorporate long stretches of walking and plenty of stairs. He takes special care to pop into Houston’s historical buildings to note their architecture, discuss the wealthy businessmen and women who helped build them, and point out artwork that displays both Houston and Texas history.

“People tell me I am very high-energy,” said Czach, whose late father once worked as a chef for the old Shamrock Hotel and the River Oaks Country Club.

After a speed walk past coffee shops through the North Travis tunnel, Czach led hikers to the JP Morgan Chase Bank Building, still known to many as Texas Commerce Bank. In the lobby the hikers stopped in front of eight preserved painted frescoes depicting scenes from Texas history, including the region’s Native Americans circa-1500 and the fall of the Alamo in 1836.

Over in the tunnel below the Esperson Building, named after the Danish immigrant who established Invincible Oil in Texas in 1905, a new photo exhibit traces its unique history. The only building in downtown Houston with Italian Renaissance architecture, it was constructed in 1927 by Niels Esperson’s wife, Mellie. “She later built a shorter annex to symbolize her and her husband standing side by side,” Czach explained.

Breezing by a mural of the first moon landing, Czach led hikers to a series of framed photos that commemorate the 1903 Jesse H. Jones Lumber Company, constructed on the corner of Main and McKinney streets, and the historic Carnegie Library and First Presbyterian Church.

In a short section under the Alley Theatre garage, Czach pointed out water damage from Hurricane Harvey and, elsewhere, the submarine doors installed in the tunnels to help prevent flooding.

As the hikers walked a particularly long stretch, sipping from their water bottles, Czach reminisced about paying $45 for scalped tickets to a Rolling Stones concert at the Sam Houston Coliseum in 1978. Tom Peacock, himself an organizer for the Houston Area Trails & More group, remembered getting David Bowie’s autograph at the Hyatt Regency Houston, where Bowie stayed during his Heroes tour.

Soon the sumptuous smells of lunch hour began to waft through the tunnels, and the group emerged to ground level, having walked 4.7 miles and 13,806 steps, according to Risley’s GPS.

Czach walked the group to the Julia Ideson Building, home to Houston’s first public library, but alas, it was closed. “Next time,” he promised.

As the hikers dispersed, Peacock marveled at Czach’s breadth of knowledge. “We share a love for the city,” he said. “That’s what it’s all about."