If you have a basement...

... you probably have a floor drain. And if you are like me, you probably have no idea how that floor drain works.  That's why I'm posting this rather prosaic information from a well-written and well-illustrated home-repair blog in the Star Tribune.

All plumbing fixtures have traps:

The purpose of a trap is to prevent foul-smelling sewer gas from coming back in to the house.  The diagram [above] shows a P-trap, which can be found at sinks, showers, and bath tubs.   The left side of the trap connects to the plumbing fixture, and the right side connects to the sewer.  The 'sewer' side will have sewer gases present, but the water sitting in the bottom of the trap prevents the sewer gases from entering the house.

Floor drains are similar... but different.

They have a "bypass" so that if they get obstructed, a tool can be inserted into the pipe via the bypass.  The bypass is normally plugged with a rubber stopper.  If it's not there, it means one of two things:

1. The drain was clogged and someone removed the cleanout plug to clean the drain, but they forgot to put the plug back in.
2. The bottom of the trap was clogged, and someone removed the cleanout plug to allow water to drain directly in to the sewer, instead of going through the trap.

If the plug is missing, foul gases can enter your basement.  More information and photos at the source.