The lower part of Porcupine Rim Trail near Moab, UT features a few approximately 2 ft wide sections alongside a 300 ft cliff, right before the trail meets the Colorado River. It's got some technical parts that require the rider's attention, putting the potential risks on the back burner of one's mind.
One of the most beautiful, yet also very dangerous trails is a 6.5 km long section of the Sentiero della Pace (Peace Trail), called Strada delle Cinquantadue Gallerie (Road of 52 Tunnels), at Monte Pasubio in Trentino, Italy. It was built by the Italian military during WWI as a bastion in the Alpine battle against the Austrian-Hungarian army.
The 52 tunnels stretch somewhere from 10 meters (33 ft) to 318 meters (1043 ft) in length, with the narrow trail winding alongside the rugged cliff.
Due to frequent deadly accidents, mountain biking on the 52 tunnel section has been prohibited for over ten years now. Hikers are still allowed, though.
Not only are most of the tunnels pitch-black inside, with low-hanging sharp-edged ceilings, there are also numerous deceptive "false" exits, with the edge of the cliff right behind it.
Despite good headlights on a bike, one's eyes will take their time to adjust to the darkness, while riding towards the light of a supposed exit. The moment one realizes the mistake, it's already too late. In the picture below, riders will be drawn towards the light when they should be turning right.
The openings used to be firing positions for the Italian artillery.
A few exits see the trail turn abruptly, also leaving no chance to those who ride carelessly into the broad light at the end of the tunnel.
I rode the trail a few times before it was closed for mountain biking. It's a lot of fun with breathtaking views, but utmost attention is required at any moment. The frequently occurring deadly accidents had been a hot topic within the MTB community for years. Finally, after another mountain biker had met his maker 1000 ft. below the rim, Italian authorities allowed only hikers from there on.
Once in a while there will be MTB enthusiasts, who lift their bikes over the barriers, in order to ride the trail illegally. During summer, somewhere around 3.45 am, right after dawn and on a weekday. The views at sunrise are stunning and the air is fresh, so I've heard. But one will go to jail and pay through one's nose when caught. Therefore, it's not worth the risk, especially since one will be in kind of an inadvertent rush in order to avoid the carabinieri. But I'd always recommend hiking, like these Italian guys did...