Imagine a Galápagos-like finger of land nearly 800 miles long—and right next door to the United States. Or picture Florida in the 1940s, before all the coastal development—but without the fresh water. These are two descriptions I'm hearing of Baja California, the arid peninsula stretching from Tijuana to Cabo San Lucas, on one side the roiled Pacific Ocean and on the other the glassy Gulf of California. There flourish a dozen species of migratory whales, herds of mobula rays—the manta's occasionally airborne cousin—leatherback and other sea turtles, a healthy billfish population despite relentless overangling, hundreds of species of birds including the elusive blue-footed booby, and numerous other indigenous, even unique, aquatic and desert life-forms.

Into this Edenic world is now stepping the full force of coastal development. The tread is heavy at Baja's tip, known collectively as "Cabo." More conscientious but problematic is tourist and residential building outside the lovely former colonial outpost of Loreto, up the east coast. And farther up yet lies a beautiful, undiscovered gem of astonishing biological fecundity, Bahía de Los ángeles, where development is still mostly rumor. What's to become of Baja is an increasingly loud debate in both Mexico and the United States. Underlying the argument is the worrisome question: Will it end up being trashed because of its unique appeal? I've come down to see the situation for myself, and to listen to those who know this spare, haunting land best.