Storm drains under the city. Started exploring them in the 90s and only stopped in recent years. Great for photography and a super fun hobby for someone who fucking hates people and likes being in total solitude lol. There's a bunch of online communities for it where I used share my photos and such. Even created a legend of sorts in Tulsa with a big ass project of painting the coordinates for every tunnel entrance in the city. It's called the drain atlas.
Anyone looking for a fun but semi risky hobby check out draining.
I explored the hell out of those drains! I grew up in Dallas. They're way more than easy enough for a person to fit through. You could fit a damn car through them. They're actually a pretty famous tunnel in the draining community. Dallas is where I started my draining career lol. I'll have to dig up some photos of that drain and show you some time. If I remember correctly we called it elm Street tunnel. It was caked in deposits as it was so old. Even had stairs in it, was really strange. Surprised I never put that together. We used to smoke weed on the grassy knoll and then explore the tunnels...