Few words have stoked the fears of ancient mariners or tickled the imagination of modern tourists quite like reef. These underwater structures, ridgelike or moundlike in shape, are most commonly associated with corals and composed mostly of those sea creatures’ skeletons. But electronic navigational gear aboard ships the world over also probes shallow waters for other sorts of reefs: those thrown up by powerful currents and composed of bottom sediments, gravel, and seashells; and artiﬁcial reefs, communities of living creatures that have established themselves on bottom debris—railroad cars, automobiles, abandoned drilling equipment, sunk boats, and the like. In U.S. waters (outside those of its commonwealth and territories), coral reefs are found off the coasts of Hawai‘i and Florida. On land in the American West, the term reef is also occasionally applied to a cliff rising above level land, suggesting a marine reef rising from the seabed—for instance, The Reef near Salida, Colorado; Castle Reef in Lewis and Clark County, Montana; and Utah’s Capitol Reef, a national park near the Fremont River and part of the hundred-mile-long Waterpocket Fold. This latter scenic region is well known for its white domes of Navajo Sandstone, reminiscent of a massive coral reef barrier and just as much a navigational impediment to travelers.