Canyons are inherently risky. Flash floods occur without notice on sunny days. Technical skills & surrounding topography knowledge required yet does not eliminate risk.
It's a secret to everybody
Overview: a 3BIII/3BIV canyon in Zion Nat'l Park with rappels up to 125 ft.
Warning: Canyoneering involves rappelling in slot canyons, always check the weather before departing and do not go if there is any chance of rain.
Make sure you take appropriate gear, which for this trip will include a helmet, harness, rappelling device, several biners, 50 ft of webbing and a few rap rings, a 200 ft rope, a 100 ft rope, and a 60 ft rope.
History: First descent unknown, Mystery has been descended by thousands of parties and is heavily permitted.
Hike: From the Weeping Rock Trailhead, head up the Echo Canyon Trail past the turnoff for Hidden Canyon, until you reach the junction with the trail towards observation point. Take the observation point trail until you reach the junction with the east rim trail. Head east on the east rim trail for approximately 1 mile until you reach the head of mystery canyon, about 1.5-2 hrs, 5 miles, and 2400 ft or so of elevation gain from your starting point. The side trip to Observation point adds maybe a half mile but will usually take an hour, so avoid this if you are behind schedule already.
The next mile will be slow going as you descend a loose dirt erosion slope affectionately known as the "Death Gully". Take your time, try to avoid causing too much damage, and avoid showering your buddies in dirt and rocks. When you finally get to the bottom breathe a sigh of relief.
Nowadays the park is trying to prevent bypassing at social trails in mystery. Canyoneers will find a few 15-20 ft rappels bolted which many will want to rappel, though a few are downclimbable. A few other 15-40 ft raps are interspersed with some hiking. After this the canyon will slot up. An 65 ft rap is shortly followed by a 50 ft rap, and after that a 30 ft rap. A few downclimbs and a few small raps will get you around the main bend of the canyon and into a shallow sculted slot. Then rap 30 ft from the old dead tree (or the bolts right next to it.)
The next section gets hot as the sun hits this most of the day. You will come upon a huge debris pile 200 ft high. This pile was formed when part of the canyon collapsed into itself A lake is said to form here early in the year. Climb up the worn trail over the debris pile and down the other side. Continue walking through the relatively flat section of canyon for awhile. Soon comes a 50 ft rap down a moderate slope.
You will reach a fluted section which will drop you into the area above mystery springs. A handline is usually setup to the exposed anchor on the left down canyon. A small bolt garden has formed here with several chains. Rig for 125 ft. 100 ft down, the rappel will place you onto a boulder, continue your rappel off the same anchor another 20 ft into the pool. There is a bolt on the back of the bolder that is seldom used. Enjoy the cool grotto (and refreshing waters) of Mystery spring. From here the canyon has a slight flow as it approaches the Narrows. Walk downstream with some downclimbs and an optional 10 ft rappel. At the end you will be 106 ft above the narrows. Enjoy the view as it's a great one. Lower your ropes (don't toss them onto the tourists) and rappel down to become a superstar. Pack it up and hike downstream to pick up the shuttle bus at Temple of Sinawava.
Water Sources: Mystery Spring, bring a filter. You won't want to drink from the narrows.
Camping: In Zion Natl park, at the South Campground or Watchman Campground.
Check out the Triplogs.