I had 3 people tell me to watch out for quick sand in the desert after a thunder store. I thought they were joking me. I figure it must be an Arizona prank they play on people that don't know any better. As it turns out they were serious about quick sand but I still did not believe them.
A mountain is solid rock and when it rains water comes down the mountain like the worlds largest house roof. Water forms a small river that runs across the desert like a river leaving what people here call a wash. A wash is where the water washes the sand away leaving a ditch about 10 ft deep 20 ft wide. When the storm is over the water is gone 30 minutes later. The bottom of the wash looks dry but the sand is full of water causing it to be quick sand.
The best way to explain it scientifically is, everything weighs less in water even sand, water takes the place of air in the soil, when you step on the soil I assume the water just squishes to the side of your shoes taking the sand with it. I assume it is a little bit like stepping in wet snow it squishes away.
After I turned off my soaker hose the soil was like quick sand for about 1 hour. After that the soil was very soft for a while maybe another hour or so. Then the soil was solid again almost as solid as dry sand. I guess it took a couple of hours for the water to go away.
This is what Wikipedia says about quick sand.
Here is what HowStuffWorks says about quick sand.
http://science.howstuffworks.com/enviro ... ksand1.htm