The Subway slot canyon in Zion National Park, Utah

© Stan Moniz/Tandem Stills + Motio

Celebrating Zion's birthday. Zion National Park, Utah

Today, we're celebrating the 102nd anniversary of southwest Utah's Zion National Park with a visit to its Subway slot canyon. Zion is defined by its breathtakingly steep red cliffs, its maze of sandstone canyons, and its waterfalls with colorful hanging gardens. Visitors here can trek for miles, ducking into crevices and resting in canyons once occupied by the Ancestral Puebloans.

The bravest souls might venture into Zion's Subway. Yes, there's a 'subway' here, but it's not a transit system or a sandwich shop. You see the image we are featuring today? That's the Subway. It's a geologic marvel, a hollowed-out canyon reminiscent of a subway tunnel.

To get here requires some technical experience, or at least a guide. There will be bouldering, climbing down waterfalls, possibly rappelling, and a 9-mile round-trip hike on a trail that is a running stream with parts so deep, hikers have to wade or swim, depending on water levels. Did we mention the water is cold? But all that work will be worth it in the end for a photo like this. Zion is the third-most-visited national park in the country, but the park grants just 60 permits a day to hikers who want to attempt a visit to the Subway.