A sinkhole on Interstate 90 just west of Issaquah is expected to cause traffic backups through the afternoon.

The sinkhole was found under the right eastbound lane of the freeway just west of State Route 900, prompting the Washington State Department of Transportation to close the right lane and right shoulder of the road.

WSDOT crews are on site and are filling the sinkhole in with concrete.

The current backup on I-90 is about a mile long, but WSDOT spokesperson Tom Pearce said that that backup is likely to grow, as filling in the sinkhole will “take most of the afternoon.”

Pearce advises drivers to avoid the area, or else to be prepared for some delays.

“Drivers need to plan ahead or take alternate routes,” he cautioned.

A sinkhole, Pearce explained, is “a void underneath the road — an open space where the soil has somehow eroded.” Such soil erosion can have many causes, such as water underground. This particular sinkhole, Pearce said, has a one-foot diameter on the surface, but underground, it is 10 feet deep with a 20-foot diameter.

“The DOT is not sure yet what caused this sinkhole, but we’re looking into it,” he said.

Sinkholes are commonly found in Florida, where they have been known to swallow entire houses; just last week, a 50-foot-deep sinkhole north of Tampa destroyed two homes and prompted the evacuation of an entire neighborhood. In Washington, however, dealing with sinkholes is a rarer occurrence for WSDOT, Pearce said.

“The DOT does not see sinkholes underneath the roadways very often,” he stated.

The Reporter will update this story when more information becomes available.