Quicksand is one of the staple environmental hazards in cartoonish low-budget movies, where it has been known to quickly swallow unwitting persons as they wander through jungles and deserts. It is usually represented as a deep pool of sandy goo which blends in with its surroundings, lying in wait to slowly suck in anyone who attempts to traverse it. Barring any nearby vines or branch-wielding comrades, the victim will slowly sink until completely submerged. Unsurprisingly, real quicksand is pretty tame in comparison to Hollywood's depiction, but traditional quicksand does have an evil cousin that would pose a grave threat to anyone who might stumble upon it... if it exists.
Traditional quicksand is created when water seeps up from an underground source and saturates an area of sand, silt, clay, or any other grainy soil. Normal sand can support extreme amounts of weight because friction between the grains of sand creates a force chain, distributing the load across a large area. But once there is sufficient moisture, the sand and water becomes a suspension where the sand particles are floating within the water. This significantly reduces the friction between the grains of sand, compromising its ability to support weight. Because the water seeps in from the bottom, the top layer of sand is often dry, causing it to appear to be normal sand.
In reality, quicksand is very rarely more than a few feet deep, making it more of a messy nuisance than a life-threatening hazard. Exhaustion is the biggest risk, considering the amount of energy it can take to untangle oneself from the waterlogged soil. When limbs are moved around under the surface, the movement creates a vacuum pressure, greatly increasing the amount of effort necessary to move. The same vacuum effect is what causes certain consistencies of mud to pull the shoes off of your feet.
In the rare instance where quicksand is deep enough that a person could become submerged in it, one would have to try pretty hard to go under. The reason for this is because the human body is a lot more buoyant in quicksand than it is in water, causing a person to float like a cork on the surface.
That's not to say that there are no dangers to becoming mired in the saturated soil. A person's buoyancy will be decreased if they are carrying a heavy pack-- possibly enough to cause one to sink-- so a quick-release backpack is a good idea in areas where quicksand is known to occur. And if a person flails around in quicksand, the sucking effect caused by the moving limbs can cause a person to sink deeper into the goo. There is also the risk of heat exhaustion, animal attacks, rising tide, and other such elemental hazards which may strike as one is working themselves free... not to mention the extremely high risk that sand will become lodged in disagreeable places.
A deep, naturally occurring area of dry quicksand would be a formidable hazard, because it would cause anyone who stepped on it to sink and become buried very rapidly. No dry quicksand has ever been officially observed outside of the laboratory, but there are reports of travelers, vehicles, and even whole caravans suddenly vanishing into the sandy earth. These reports have always been viewed as mere folklore, but perhaps there is more to the stories than we realize. Science does not completely dismiss the possibility of naturally occurring dry quicksand; in fact, during the planning of the Apollo moon missions, scientists added large plates to the ends of the Lunar Module legs to help support the craft in case the astronauts found dry quicksand on the moon... but the precaution proved unnecessary, since no such soil was encountered.