When Jaan Roose, 34, a three-time slackline world champion from Estonia, stepped onto a line 120 meters above downtown Seoul on Thursday, a collective gasp rose from the hundreds watching below. "Seeing a person up there feels like watching a video. It doesn't seem real," said Sung Na-hyun, 37, an office worker, adding that it was her first time watching a slackline performance. Sponsored by the Seoul Metropolitan Government and hosted by Red Bull, the event was the first slackline performance held high above central Seoul. Slackline is an extreme sport in which athletes walk along a line about 2.5 to 5 centimeters wide stretched between two points, performing tricks such as jumps and spins using the line's sway and rebound. Roose, who began slacklining at 18, became the first person to slackline across the Strait of Messina, which separates Sicily from mainland Italy, in July 2024. Two months later, he became the first to cross from Asia to Europe on a slackline, walking 1,074 meters over the Bosphorus in Istanbul, Turkey. He kept breaking new ground in January 2025, walking a 100-meter