Fathoms deep, a diverse, endangered world

Ninety-nine percent of the biosphere is ocean.

And about 85 percent of that is beyond the reach of sunlight. So, by far, the most common place to live on earth is not desert or rain forest but in the almost incomprehensibly vast, cold, and fluid environment that is the deep sea.

For pretty much all of human history, we've assumed the deep was devoid of life. Plato concluded the sea bottom was "corroded by the brine, and there is no vegetation worth mentioning, and scarcely any degree of perfect formation, but only caverns and sand and measureless mud."

For millennia, no one bothered to disagree. Who would? Deep water is dark, cold, and food-poor. And pressure increases as you descend: 2 1/2 miles down, the pressure exerted on a square inch (picture your big toe) is 5,880 pounds (picture two Volvos on your big toe). Oceanographers are fond of putting Styrofoam cups in socks and attaching them to deep-water instruments. Sink a cup to 10,000 feet, and it will come back to the surface the size of a thimble.

What could possibly live under those circumstances?

Plenty, it turns out. "Untold billions of organisms," as one oceanographer puts it. The deep oceans are host to a breathtaking range of life: Perhaps as many as 30 million species, from the black-devil angler fish to the benttooth bristlemouth, the most abundant vertebrate on the planet; from 40-foot squid with eyes the size of human heads to microscopic bacteria thriving in the gills of deep-water clams.

There are coral reefs a quarter-mile from the surface. There are vast feeding communities swirling above underwater mountaintops. The diversity of animal life on the deep seafloor alone, estimates one marine biologist, "may exceed that of the Amazon rainforest and the Great Barrier Reef combined."