Why is it so hard to find flight 447's black boxes?

BEA images
Debris scattered over quite a small area (600 m x 200 m) at 3900 m depth
which seem indicating that the plane impacted the ocean in one piece

From PhysicsCentral

The French government announced today that more bodies and wreckage from the Air France flight 447 crash off the coast of Brazil had been found almost two years after the crash.
The critical pieces of the puzzle, the plane's flight data recorders - or black boxes - are still missing, however.
But why are they so hard to find?

The wreckage from flight 447, which crashed in the Atlantic Ocean off the coast of Brazil in June 2009, is scattered over mountainous ocean floor at depths from 12,500 - 13,000 feet, or around 2.5 miles.
Though that doesn't sound very deep, especially when you consider that modern airliners often cruise at three times that height, it's far too deep for scuba divers or naval submarines to explore.

For every 33 feet you descend under water, the atmospheric pressure pushing down on you increases by 100 percent.
At the surface, every person and object has 14.7 pounds per square inch of air pressure pressing down on him or her.
That pressure feels normal to us, but once it is increased or decreased, it starts to cause problems.

Boyle's law tells us that as the pressure doubles, the volume decreases by half (assuming there's no change in temperature).
If, for example, we pulled a party balloon under water, at 33 feet - where the atmospheric pressure is doubled - the balloon would be half as big as it was at the surface.
Drag it down another 33 feet and it would shrink in half again.
Human's lungs are a lot like a balloon.
At around 100 feet, humans reach the limit of how much pressure their lungs can stand.

Imagine what it is like at 12,500 feet, where the wreckage is scattered.
Even modern naval submarines can't stand the pressure at that depth.
They call it a day at around 1,000 feet where the pressure is over 30 times the surface pressure. Beyond that, they could be crushed like an empty soda can.

Accident site (in the area of an abyssal plain) :
Note the position in the middle of the previous search zones on a flat and sandy seabed
around 5 NM, north of the last known position (LKP : 2°58.800 N, 30°35.400 W)
--> localization with the Marine GeoGarage <--

At the wreck site, the pressure is almost 400 times the pressure at the surface.
To survey the sea floor at 12,500 feet below the surface, a special mini submarine called a submersible is needed.
Submersibles have a limited range and can only stay under water for hours at a time so they usually work in conjunction with another vessel - a ship or a larger submarine.

A yellow submersible called Nautile, who previously surveyed the Titanic wreckage, helped originally locate flight 447 wreckage in 2009. (The Titanic wreckage is also at a depth of about 12,500 feet.) A similar sub, called Alvin, was also used to explore the Titanic wreckage when the ship was discovered.
Though it seems impossible, the tiny sub is protected by a titanium pressure hull just 2 inches thick. Both subs carry 3 explorers.

The sub that found the latest wreckage was an unmanned sub called a REMUS 6000.
The REMUS 6000, with no human occupants, can travel a little faster and a little longer than Nautile or Alvin, but even it can explore for only 22 hours at a time, limited by battery life.
(see BEA phase 4 mission preparation)

A deep, dark environment and the special tools needed to explore it make searching for anything at 2.5 miles under the sea a difficult task.
There's hope, after finding an engine and landing gear, that the black boxes may yet be found, and help solve the mystery of what really happened to flight 447.
Even if they are found, it's anyone's guess whether the boxes will have survived two years of crushing pressure in a corrosive seawater environment.

Links :

• BBC : Wreckage from Air France jet found in Atlantic
• NYT : Bodies from 2009 Air France crash are found
• TheGuardian : Air France plane crash victims found after two-year search
• CNN : Father of crash victims: Leave their remains under water
• BEA : Flight AF447, sea search operations, phase 4
• WHOI : WHOI-led team locates Air France wreckage