One specific thing is confusing me. The tunnels are said to be 200 years old, but the video shows narrow concrete passages. Concrete as a widely used building material didn’t take off in the US until well after 1823, which is 200 years ago. Portland cement was only invented in the 1820s, and large-scale use for underground structures came much later. How do you have a 200-year-old concrete tunnel when the technology barely existed back then? That detail doesn’t line up. I can’t wrap my head around the timeline here.