Arts and Entertainment your connection to The Boston Globe

Fathoms deep, a diverse, endangered world

The Silent Deep: The Discovery, Ecology, and Conservation of the Deep Sea
By Tony Koslow
University of Chicago, 270 pp., illustrated, $35

The Deep: The Extraordinary Creatures of the Abyss
By Claire Nouvian
University of Chicago, 252 pp., illustrated, $45

Any world map shows that the earth's surface is about 75 percent ocean. But it's easy to forget that the seas are, on average, 2 1/2 miles deep. On land, animals live on the ground or within about 200 feet of it. In the ocean, animals live at the surface, or at the bottom of the deepest trenches, 6 1/2 miles down. Multiplied out, that means 99 percent of the space inhabitable by the earth's animals exists in the oceans. That's worth repeating.

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."

Two new books from the University of Chicago should help forever banish the paradigm of the lifeless deep. Tony Koslow's "The Silent Deep " is an illustrated survey of deep-sea ecology, deeply informed by history and rendered in straightforward, careful prose. Claire Nouvian's "The Deep " is a big, glossy book of deep-water photographs, punctuated with short essays by 15 leading oceanographers. (Koslow has an essay in Nouvian's book. )

The two books present earth's biggest, strangest ecosystem with reverence and wonder. Koslow tells the stories of deep-sea pioneers like Wyville Thomson and William Beebe ; tours us past hydrothermal vents, underwater mountains, and whale falls; and laments the destruction of deep - water habitats caused by mining, pollution, and bottom trawling.

Nouvian's "The Deep" features more than 200 color portraits of the planet's least-known creatures: sparkling pink octopi like floating lanterns; iridescent squid with corkscrew tails; predatory fish with hooded eyes and translucent teeth looming in the darkness. Some of these are the first-ever photographs of certain organism s. At least eight of the pictures feature animals so unknown that Nouvian's captions list them as "unidentified."

To page through her book is to feel as if you are peering at life on another planet: It is a vision of the deep as a vast, balletic swarm of nature's inventiveness. Ninety percent of deep-sea animals create their own light, and Nouvian's pictures capture the eerie beauty of bioluminescence especially well. Excepting perhaps microbial life inside the earth's crust, deep water remains biology's darkest frontier. We have better maps of the topography of Mars than we do of the seafloor. "For the last twenty-five years," Nouvian writes, "a new deep-sea species has been described every two weeks, on average."

We know now that deep-sea environments are not the abiding oases one might imagine . Commercial fisheries have devastated the populations of deep-water fish like oreos or orange roughies, animals that can live more than 100 years and take 20 or 30 years to reach sexual maturity. The hunting of great whales has not only decimated whale populations, but also diminished the unique, startlingly rich habitats created when whales die and sink to the sea floor. Cold-water corals that grow perhaps an inch a year, in reefs up to 10,000 years old, are being wiped out by bottom trawlers. "Today," Koslow writes, "virtually no part of the deep-sea fauna remains unaffected by man's activities."

At the bottom of the ocean, Plato assumed, "nothing is in the least worthy to be judged beautiful by our standards." One look at Nouvian's "The Deep" should correct that assumption forever.

Now if we can just preserve the earth's most extensive ecosystems long enough to understand them.

Anthony Doerr is the author of "The Shell Collector" and "About Grace."