what is exactly is quicksand?

Question

and how deep can it get? approximately how many people die a year from it?

Answer 1

Quicksand is sand suspended in water - it's like a very thick water, it can get awfully deep, and it's really easy to panic and drown. It's also a relatively simple maneuver to swim out of it, although it's tiring.

Learn more below:
http://en.wikipedia.org/wiki/Quicksand

Answer 2

Quicksand is a hydrocolloid gel consisting of fine granular matter (such as sand or silt), clay, and salt water. The origin of the name refers to "quick" in the older meaning of "alive" rather than "fast," and is thus similar to the origin of the term quicksilver for mercury.

When undisturbed it often appears to be solid, but a minor (less than 1%) change in the stress on the quicksand will cause a sudden decrease in its viscosity. After the initial perturbation—such as a person attempting to walk on it—the water and sand in the quicksand separate and dense regions of sand sediment form; it is because of the formation of these high volume fraction regions that the viscosity of the quicksand seems to suddenly increase. In order to move within the quicksand, a person or object must apply sufficient pressure on the compacted sand to re-introduce enough water to liquefy it. The forces required to do this are quite large: to remove a foot from quicksand at a speed of one centimeter per second would require the same amount of force as "that needed to lift a medium-sized car." [1]

It was commonly believed that the behavior of quicksand was due solely to saturated or supersaturated suspensions of granules in water. Pressure from underground sources of water would separate and suspend the granular particles, reducing the friction between them. As of September 2005, it has been shown that it is the presence of salt that is largely responsible.[1] The stability of the colloidal quicksand is compromised by the presence of salt, increasing the likelihood of sand flocculation and the formation of the high viscosity regions of sediment responsible for quicksand's "trapping" power.

Quicksand is not as dangerous as depicted in many movies. As quicksand is rarely more than a few feet deep, there is usually little danger of sinking below the surface. Even when the quicksand is deep enough, deliberate effort is required to sink below the surface. Quicksand is typically denser than the human body, meaning that a body is much more buoyant in quicksand than in water. Thus, the body will float quite easily in quicksand.

This was demonstrated in an episode of the TV series MythBusters and in an article in Nature. [1] The sand's higher density will gradually push a human body upward, eventually allowing one to paddle toward more solid footing. Typically the greatest danger of getting stuck in quicksand comes from exposure, starvation, flash flooding, or tidal flooding.

Quicksand can be found inland (on riverbanks, near lakes, or in marshes) or near the coast. It can also form when an earthquake increases groundwater pressure, forcing the water to the surface and causing soil liquefaction.

One region notorious for its quicksands is Morecambe Bay, England. As the bay is very broad and shallow, a person trapped by the quicksand would be exposed to the danger of the returning tide, which can come in rapidly.