Back in the day I used to scuba dive. To get my certification I had to take classroom sessions regarding the science of diving. My instructor pounded many things into our heads, one of which is the importance of pressure because water pressure can kill you slow or in no time flat. Take your pick - it just depends on how stupid you want to be.
When you are at sea level, there is a whole lot of air over your head; about 62 miles of it (100 kilometers). At this height the air is so thin that it is considered a vacuum. I use sea level because the higher the elevation, the less amount of air there is above your head. Where I live we are 390 feet above sea level. That means that I have about .0175 pounds less pressure on my head than if I would if I were vacationing at a beach in Miami.
The air we breathe weighs something because even gases are made from atoms that weigh something. Air is comprised of nitrogen (78%) and oxygen (21%) with traces of argon (1%) and carbon dioxide (0.03%) - and of course water vapor.
What does it weigh? Those 62 miles of gas above your head weighs 14.7 pounds per square inch (about 1000 millibars, a millibar is 100 Newtons per square meter). So, if the top of your head has ten square inches supporting a 62 mile high column of air above you while standing, your neck is supporting 147 pounds of pressure that is trying to drive your hairy coconut into your chest. Plus that coconut above your shoulders weighs about 12 pounds; so make it roughly 160 pounds of pressure your neck, spinal column, legs and feet are withstanding just due to air pressure and the bony/brainy structure that your ears hang from.
That 14.7 pounds of pressure is known as an atmosphere - one atmosphere to be exact. And the pressure exerted by 62 miles of air is equal to 33 feet of water. Lets extrapolate.
BP originally applied to drill in 500 feet of water off the Louisiana coast. The State of Louisiana approved this permit but it was then denied by the feds who told BP to move it out into deeper water - 4,800 feet deeper. What's the pressure differential?
The original permit had the well head at a depth of 500 feet - which is 15 atmospheres. The new federal requirement put the well head at 5,280 feet, over ten times deeper and under a pressure of 160 atmospheres. The pressure differential is this - at 15 atmospheres a diver or any equipment would have to operate in an environment of 223 pounds psi. At 160 atmospheres it changes to 2,352 psi. No way a diver can live under this kind of pressure without a strong steel exoskeleton framework to resist the pressure. There are only eight people who ever dove below 800 feet - and lived. Our most modern submarines cannot operate safely at less than half the depth of the Deepwater Horizon well head. The crush depth of the SSN Seawolf is estimated at 2,400 feet.
This means that everything must be handled via remote controlled machinery - robots, diving platforms, the Nautilus and Capt. Nemo - whatever. We can watch the sucker spew out thousands of barrels of crude every hour but we can't physically lay a hand on it.
Trying to snuff a leak at 52 hundred feet is a lot different than maneuvering a Mars robot around a rock 86 million miles away. The pressure behind the Gulf blow out must be truly horrendous.
And the government forced BP to drill at this depth.