The Subway in Zion NP

The Subway in Zion National Park is considered a semi technical canyon, requiring some short swims, a bit of rock scrambling, a few short rappels, and lots of wading in shallow water.
Novices (like myself) should attempt this with those having canyoneering skills. The rewards for these efforts are views of sublimely carved canyons which give the sense of having entered into a space intended for someone else.

For a more easily accessible slot canyon The Narrows in Zion is an excellent destination. - Martin van Hemert