Travis Kelce has a vault of his own.

The Kansas City Chiefs tight end, 35, has a hidden box tucked deep down in SubTropolis, a massive cave 150 feet below the ground preserving items that are significant to his standout NFL career.

The zinger? Kelce didn’t even know this collection of valuables existed until he was introduced to them when filming for the football franchise’s recent ESPN docuseries, The Kingdom, directed by Kristen Lappas.

In the first episode, Kelce, Chiefs quarterback Patrick Mahomes, defensive tackle Chris Jones and head coach Andy Reid are intimately captured walking underground within the 1,260-acre space that was created through the mining of a 270-million-year-old limestone deposit.

“I’ve been here for eight years and have never even known that this existed,” says Mahomes, 29, in the series, while Kelce compared the dark, mysterious ambience to a scene out of the Indiana Jones films, candidly voicing, “This is crazy.”

Following the release of The Kingdom on Aug. 14, PEOPLE had an exclusive look inside SubTropolis to explore the underground space and the Kansas City Chiefs archives that lie within the caves guided by team historians Bob Moore and Mike Davidson.

The items chosen to be archived within SubTropolis — which was developed by Lamar Hunt, founder of the Kansas City Chiefs and the American Football League (and father of Clark Hunt, current owner and CEO of the franchise — must be “oddities,” Davidson tells PEOPLE.

They’re not looking for just another football, the historians emphasize, which is why Kelce has among the most obscure — yet significant — items preserved in his personal box: dirt.

It’s not just any dirt, however. Rather, it’s the dirt that stained his Chiefs jersey from the catch he made during the last NFL football game played on an infield dirt baseball field in Oakland, Calif., against the Raiders on Sept. 15, 2019.

“So I had the idea … I said, ‘This would be kind of cool if we got some of that infield dirt from there,’ ” Davidson recalls from the game that was the last time the Raiders played on the dual-purpose Oakland Coliseum while the A’s baseball season was still ongoing.

“So we did,” he continues, explaining, “But what ended up happening was, Travis caught a pass in the infield dirt and we secured his jersey … We have the infield dirt in there.”

Davidson, who’s entering his 43rd year with the NFL, came up with the idea to preserve the infield dirt because “we played there a bunch of times,” he says. “It’s a big deal, because years ago you shared this field with the baseball team,” he explains, adding, “You shared the locker rooms.”

Also in the Ohio native’s tucked-away collection is the suit jacket Kelce wore for his Saturday Night Live monologue after the Chiefs won the 2023 Super Bowl. Kelce made his hosting debut on the NBC comedy show in March that year.

There’s often a lot of overlap with the artifacts that are contained within SubTropolis and the Hall of Honor at GEHA Field at Arrowhead Stadium in Kansas City.

“Some of it we took out the Hall of Honors,” Davidson shares of the underground archives. Moore adds, “Some of it’s on display, so we couldn’t take it out and put it in a box because already on display at the Hall.”

As for Kelce’s thoughts on his underground archives, as expressed in The Kingdom, “This is some good stuff right here,” he says in episode six when taking his first look inside, adding, “Looking to add to the box. I don’t know about you guys.”

The Kingdom chronicles the Chiefs’ 2024 season and deep dives into the franchise’s distinctive place in the NFL’s landscape. It’s produced by Words + Pictures in association with Skydance Sports, NFL Films, 2PM Productions and Foolish Club Studios.

“The real magic and heart of the series comes from the players, coaches, and executives who trusted us; their candor shows fans how the dynasty came to be, and why the Chiefs are such a singular organization,” Lappas said in a statement.

Chiefs CEO and Chairman Clark Hunt called The Kingdom a “tribute to everyone has helped shape” the franchise over the past 60 years.

ESPN’s Chiefs Kingdom is airing now and available for streaming on ESPN+.