If you'd like a video I can endorse, here's a good Tim Traveler video about the largest stadium in the world (now abandoned). Things of interest here:
1. I am entirely captivated by the abandoned piano at 2:35, evolved into a sort of permanent planter like the art car that lives in Toronto's Kensington Market
https://www.youtube.com/watch?v=jF0rGhFf0c4&t=145s
2. The British pronunciation of "Funicular"
3. I am now very curious to see someone do a history of athletic self-improvement orgs being oppressed by governments

Consider:
- Sokol, early 20th century, Czechia
- Falun Gong, late 20th century, China
Both organizations that are fundamentally about outdoor calisthenics, their activities expanded and/or the government viewed their sheer size as a threat, and the Nazis/Communist Party of Czechoslovakia/Chinese Communist Party oppresses them.
How this fits into the Boxer Rebellion, where boxing self-improvement clubs *did* try to overthrow the government, you'd have to uncover through research.