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Fathoming an ocean's depth; extreme peripheral vision.

Q: How deep are the oceans? How deep is a fathom? (Tom, Miami, Florida)

A dweller at the bottom of the seas: the sea cucumber feeds on small plants and decayed debris that settles into the seafloor sand and mud. [©Larry Madin, Woods Hole Oceanographic Institution]

A: The ocean plummets to 6.8 miles (11.0 km) at its deepest and averages 2.4 miles (3.8 km), not including seas that don’t connect to the ocean, such as the Caspian Sea.

The oceans form a contiguous body of seawater. The blue and other colors show the water and the black illustrates the separating land continents. [National Oceanic and Atmospheric Administration (NOAA)]

Continents and archipelagos divide the salt water into five bodies: the Arctic, Atlantic, Indian, Pacific, and the Southern (formerly, the Antarctic) Oceans. The Pacific — with its profound Mariana Trench — is the deepest (2.7 mi, 4.3 km average depth). The Arctic — with its jutting continental shelves that underlie one-third of the ocean — is the shallowest (0.6 mi, 1.0 km, average depth). See the table below for depths of the various oceans.

Sea depth varies greatly due to rugged submarine topography. Chains of mountains snake between and around continents. Seamounts loom in parade-like lines — conical volcanoes, some active, others not. Valley-like basins, plains, and plateaus stretch between the mountains. Trenches delve to great depths — six times deeper than the Grand Canyon. The scale beneath the sea dwarfs mountains and valleys on the land.

We didn’t start mapping the sea until the 1870s. Then, steaming along, we transmitted acoustical pulses down to the ocean’s floor and timed how long the pulses took to reach bottom and reflect back. From the elapsed time and the speed of sound in water, we calculated the sea depth. At the rate research vessels crawl (14 mph, 22 km/h), it would take about 125 years to finish the job.

Now, we zip overhead. Altimeter probes from satellites have mapped most of the seafloor by examining the broad bumps and dips on the ocean’s surface that mimic the rises and valleys that comprise the ocean’s floor.

To answer your last question: a fathom is a depth of six feet (1.8 m).

Further Reading:

NOAA: Ocean

National Geophysical Data Center: Global Seafloor Topography from Satellite Altimetry

Wikipedia: Ocean

Q: Can a chameleon or spider see behind itself without turning its head? (Parvin, Singapore, Singapore)

A: Yes. Both can!

Chameleons have eyes mounted in tiny turrets they swivel independently. Each can scan a 180-degree arc from front to back. So, he can see behind with either eye while holding his head and body utterly still. Image.

Jumping spiders can see almost 360 degrees about them — up, down, and all around. They have four pairs of eyes that look in three different directions.

A dragonfly zips about and lands behind a jumping spider. Her side eyes detect the bug’s motion. It’s a blurry image but just clear and wide enough to see the bug behind her.

A little jumping spider grabs her huge prey like a tiger. [© Ed Nieuwenhuys]

Lunch! She whips her body around and locks onto the moving prey with her large middle-front eyes. She focuses the image sharply and brings it in closer — a clear telephoto image in color. The flashy green iridescent bug fills her retina.

She stalks like a tiger closer and closer. Advancing, she judges distance with her side-front eyes. Close enough — about an inch (2.5 cm) away. The small spider leaps — 20 times her body length — on the enormous dragonfly and grabs it with her jaws.

Further Reading:

Ed Nieuwenhuys: Jumping spiders (family Salticidae)

discovery.com: Lizards

Australian Museum Online: How spiders see the world

British Journal of Ophthalmology: A roving eye

Q: I know that the earth's surface is 7/10ths water but what percentage of each ocean takes up that 70%? (Morgan, Marietta, Ohio)

A: The total global ocean (including adjacent seas) covers about 152 million square miles (394 million km²). The Pacific is the biggest ocean. Its area exceeds the surface of all our continents combined. The Atlantic is second in size and drains a land area four times greater than the Pacific does. The third — the Indian Ocean — is the youngest ocean, formed a mere 80 million years ago after the southern supercontinent, Gondwana, http://en.wikipedia.org/wiki/Gondwana broke up. Please see the table for percentages (calculated using ocean areas that include adjacent seas).

(Answered April 15, 2005)