This is a video of intrepid explorer and Youtuber post 10 descending a steep 10%+ grade mountain runoff culvert, which leads to a waterfall at the end. It takes him about 17 scary minutes to reach the waterfall, at which point he immediately turns around and heads back up instead of yelling "Geronimo!" and cannonballing down the fall like I'd been hoping. He does walk around and show the waterfall from the outside though around 31:00. Me? I would have just ridden the whole thing in an inflatable pool float like the world's deadliest log flume. "You would have died." Pfft, you can't kill what's already dead. "For the last time, you're not an undead warlock." It says so on my driver's license! "This is a D&D character sheet." Well it's the closest I've got.
Keep going for the video, but remember he approaches the very bottom around 16:00, and shows the bottom from the outside around 31:00.
Thanks to my friend becca b, who agrees at least the Ninja Turtles were smart enough to not live in that pipe.