Dirt

How much do you really know about the dirt beneath your feet?

First, when in a scientific or engineering context, we call it soil - just one of those things.

Second, soil can behave in some interesting and complicated ways, mainly because it is a mix of solid, liquid, and gas: minerals and organic materials, water, and air.

The mineral (rock) portion of soil is generally broken down by particle size, because particle size has the biggest effect on how they behave. How big the particles are is just how finely the original rock mass has been weathered down by erosion and other forces.

Boulders	On the scale of a refrigerator	Unsurprisingly, behaves much like solid rock
Cobbles	Think softball or melon	Mostly found in transition zones mixed with other types. What types they are will mostly determine behavior.
Gravel	About an inch, like you see on a path	Relatively strong and stable. Large void spaces between grains makes it very easy for water to flow though. Good for drainage.
Sand	About a millimeter, like on a beach or sandbox or sandbag	Similar to gravel, but smaller size makes it susceptible to acting like a liquid when saturated and under certain conditions
Silt	Micron scale, think milled flour	Like clay, but not as extreme
Clay	Very small, around smoke particles	At this scale, things act in strange ways. When dry, it could float in the air heedless of gravity, or be part of a hard mass of stone or pottery-like material. At various levels of liquid content it can dramatically shrink or swell. It can be sticky, but it can also be very useful as a lubricant. It will aggressively soak up water, but once filled can serve as an impervious barrier.

Clay will shrink dramatically when it dries and swell again, with water.

Typically, the coarsest soils will be deepest, with finer soils closer to the surface. So, if you were to drill down, you will likely find a series of layers of similar material.

1. First, perhaps some soil with lots of organics in it. This is usually dark in color. At the extreme, you'd call it potting soil. Contains things useful for life: decaying plant and animal matter, bugs and worms, roots, etc. This is what people really think of as "dirty." This is usually rather shallow, some feet, or nonexistent.
2. Next silts and clays. Often mixed with sand. This layer can be over a hundred feet deep, but is usually much less, or nonexistent.
3. There is almost always a layer of sand and gravel, usually with some clay and silt content. Could be anything from zero to hundreds of feet thick.
4. Then a transition mix from sand, gravel, and cobbles to boulders and solid rock mass. This is always down there somewhere. In the mountains it's often the surface.

Remember there is space between all these particles of rock of whatever size. Water can reside in and flow through these spaces. Almost anywhere you are standing over water. The depth where you can find it is called the water table. That depth is not uniform, so the water underground is always flowing from higher to lower areas. Think of a surface river. Much of that water is flowing over the surface, which is what you see and think of, but much is also soaking down into the ground. So at the edge of the river the water table is at zero depth, and water is flowing down toward deeper areas. Also water in the ground at high elevations (say in hills or mountains) is flowing down toward the valleys, rivers and oceans. 

Have you ever wondered how a river continues to flow so long after a rain? It would seem that all that water hits the ground at the same time and should rush off down the hill in a torrent and be gone. Well, most of it actually goes down into the ground. Then, underground, flows very slowly toward the river and downhill. The rain soaks the soil, and the soil feeds the river over days and weeks.

Wells work on a similar principal. You send a shaft down below the water table and the water in the surrounding soil flows into this hole you've created until it's full up to the level of the table. Then you can take water out of the hole and it will refill.