Kansas City Chiefs quarterback Patrick Mahomes had an interesting reaction to Travis Kelce’s vault in the franchise’s subterranean archive. Captured as part of "The Kingdom," an ESPN docuseries on the Chiefs’ run to Super Bowl LIX, the "SubTropolis" surprised Mahomes.One of the many discoveries in the six-episode docuseries is an underground business complex that holds a lot of the Chiefs’ treasured history. Developed by the Chiefs' founder, Lamar Hunt, it sits on a space of 1,260 acres, acquired by mining a 270-million-year-old limestone deposit.On entering the business complex 150 feet underground, the Chiefs’ quarterback could not hide how impressed he was. He said:“I’ve been here for eight years and have never even known that this existed.” View this post on Instagram Instagram PostMahomes was drafted by the Chiefs with the 10th pick of the 2017 NFL draft, after three years with the Texas Tech Raiders. The three-time Super Bowl champion became the team’s primary quarterback after playing backup to Alex Smith in his rookie season.In seven seasons as a starter, Mahomes has led the Chiefs to seven consecutive AFC championship games, going to the Super Bowl five times. His three Super Bowl wins, including two consecutive ones, made him the fifth quarterback to win the championship three times as a starter.What is in Travis Kelce’s Chiefs’ underground vault?According to a report by PEOPLE, one of the items preserved in the Chiefs’ underground vault is dirt from a Travis Kelce jersey. It bears significance, not just in the franchise’s history, but also in the history of the NFL.The dirt came from the tight end’s jersey from a game in 2019. It was the last NFL game played on an infield dirt baseball field in Oakland. The dirt had stained Kelce’s jersey while making a catch during the game against the Raiders.The Raiders would subsequently move to Las Vegas and start playing in Allegiant Stadium. A Kansas City Chiefs historian, Mike Davidson, told PEOPLE how the dirt came about. He said:“So, I had the idea … I said, ‘This would be kind of cool if we got some of that infield dirt from there.’ So, we did. 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.”This season will be the 10-time Pro Bowler’s 13th NFL season.