If you've gone outside over the past few weeks, you may have noticed the smell of smoke in the air.

Virginians from Virginia Beach to Roanoke and north as far as the Washington suburbs have smelled the smoke from two fires. One is burning in the Great Dismal Swamp in southeast Virginia, but most of the smoke is from a fire (the Evans Road Fire) burning in the wetlands of eastern North Carolina. That fire was started June 1 by a bolt of lightning.

You might be wondering, "How can wetlands burn?"

The conditions for these fires were set up by the very dry conditions that Virginia and North Carolina have been experiencing for more than a year.

Wetland soil contains a lot of plant material. It is similar in some ways to the peat moss that you might use in your garden or for your houseplants. When wetlands dry out during a drought, the soil can become fuel for fire. That's what happened in North Carolina.

When soil begins to burn, there are two big problems.

First, soil fires produce a lot of smoke. Smoky fires happen when there is not enough oxygen for the burning material to be entirely consumed.

Smoke, especially the kind you can smell, is basically unburned fuel. Because the soil fire is burning underground, there is not a lot of air and, so, there is a lot of smoke.

The second problem is that soil fires are very difficult to put out once they start. They can be extinguished only by soaking the ground thoroughly with water. Firefighters simply can't do this. As soon as they soak one area and move on to the next, the soil will begin to dry and the fire will start up again.

Even though the North Carolina fire has been contained, it will continue to burn in place until all of the organic matter in the soil has been burned or until there is a sustained, heavy rain over the entire area.

Some officials believe it will take several inches of rain from a tropical storm to extinguish the fire.

There may be some consolation in that the North Carolina fire started naturally and is part of a natural cycle in those wetlands. But that certainly doesn't make it any easier to breathe. So watch out for smoke alerts and pay attention to any air-quality warnings.

Correlated Virginia science Standards of Learning: 2.6, 3.7, 3.10, 4.8, 6.6, LS.11, L.12, ES.3.
James S. Beard is curator of geology at the Virginia Museum of Natural History in Martinsville.