STRANGE as it sounds, we know more about distant planets than we do about the deep sea.

Light-years may measure greater distances, but fathoms can be  unfathomable.

Inky darkness, icy temperatures and, most of all, crushing pressures conspire to make deep exploration daunting if not impossible. That is why scientists estimate that humans have glimpsed perhaps only a millionth of what there is down there to see.

That also helps explain why a runaway oil well on the seabed in the Gulf of Mexico is a massive calamity, with crews struggling to stop the gushing crude. It is the brutal nature of the abyss.

The biggest challenge is water’s inherent density. Accumulations crush all but the strongest gear. For every 33 feet of seawater, the pressure exerted on an object increases by the equivalent of what a human experiences at sea level  14.7 pounds per square inch. At the wellhead, roughly a mile down, that translates into a pressure of more than one ton per square inch.