• Question: why would we be crushed at the bottom of the ocean if we didn't have equipment like submarines?

    Asked by baeby to Chris, Josh, Rebecca, Rob, Susan on 22 Jun 2015.
    • Photo: Rebecca Dewey

      Rebecca Dewey answered on 22 Jun 2015:

      Any fluid that sits on top of something else, causes pressure to the thing below it. This is true of air and water. The more depth of water on top of something, the more pressure that will be exerted onto the object due to the weight of all the water on top. Unless the pressure is equalised on the inside (i.e. by increasing the air pressure inside it), this will cause the object to be crushed.

    • Photo: Susan Cartwright

      Susan Cartwright answered on 22 Jun 2015:

      Essentially, because water is heavy.

      If you are sitting on the land, you have a column of air pushing down on you, creating air pressure (about 100000 N per square metre). Air has a density (at sea level) of about 1 kg per square metre.

      If you are underwater, the weight of the water is pushing down on you. The density of the water is 1000 times more than the density of air, so it only takes a relatively small depth of water to make a big change in pressure: at 10 m down, the pressure is already double what it was at the surface.

      Some parts of your body can equalise the pressure. For example, a SCUBA diver breathes air at the same pressure as the water around her – at 10 m, she will be breathing air at twice atmospheric pressure (and this means her air tank will only last half as long as it would if she were just lazing along a couple of feet below the surface). Therefore it does not get harder to breathe when you are diving. However, the increased pressure does cause gas to be absorbed into your tissues, and if you come up too fast, this will bubble out into your bloodstream. Bubbles in the blood affect blood flow, so this is dangerous and sometimes fatal (it is a condition called “the bends”). Divers who have to come up fast because of an emergency (e.g. an accident to their air supply) are rushed to a hyperbaric chamber, which is a sealed room where extra air can be pumped in to match the pressure they were at. The pressure is then reduced very slowly to allow the gas to be released gradually without forming large bubbles.

      However, there are some spaces in your body, for example cavities (“sinuses”) in bones, which do not communicate with the outside and so cannot equalise pressure. If you were to go to too great a depth, your bones would be crushed. Deep-diving whales have many special adaptations to prevent this happening to them.

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