"r/Route66 Recommends..." Favorite Natural Site Edition

As part of the “r/Route66 Recommends…” series, what is your Favorite Natural Site along the Route? Anything that is Natural is allowed! (i.e. Not Man Made)!

With plenty of caves, deserts, mesas, and more, there's plenty of natural beauty along Route 66! Let r/Route66 know which one(s) is/are your favorites!

Thanks!