Chris Renzelman, age 11, of Tulsa, Okla., for his question:

What exactly is sewer gas?

Sewers carry off waste materials  and organic wastes tend to decay. In this decaying process they break down into simple chemicals of hydrogen and carbon. Some chemicals are freed as gases and go off to mingle with other gases of the sir. One of the most common decay gases is methane. It hovers over marshes and stagnant swamps of rotting vegetation. It rises from the rotting wastes in a sewer.

Methane is an invisible and odorless gas. It is a hydrocarbon chemical and highly flammable. sometimes it forms in mines where decaying organic materials have been trapped underground for long periods. There it becomes part of a mixture of gases called fire damp that may explode from a mere spark. Trapped sewer gas also may explode when sparked. Underground sewers, however, have plenty of built in vents to let it escape before it builds up in dangerous amounts.