photos by PAUL EFIRD/NEWS SENTINEL Crews repair a sinkhole Wednesday on Alcoa Highway. The sinkhole has closed all northbound lanes and two of three southbound lanes, according to Tennessee Department of Transportation officials. The sinkhole is just south of the J.E. 'Buck' Karnes Bridge.

SHARE A sinkhole has closed all northbound lanes and two of three southbound lanes of Alcoa Highway near Cherokee Trail. (TENNESSEE DEPARTMENT OF TRANSPORTATION) A sinkhole has shut down northbound Alcoa Highway and two of three southbound lanes near Cherokee Trail. (KNOXVILLE POLICE DEPARTMENT) Crews repair a sinkhole Wednesday on Alcoa Highway. The sinkhole has closed all northbound lanes and two of three southbound lanes, according to Tennessee Department of Transportation officials. The sinkhole is just south of the J.E. 'Buck' Karnes bridge. (PAUL EFIRD/NEWS SENTINEL) Southbound traffic is down to one lane near a sinkhole Wednesday on Alcoa Highway. The sinkhole closed all northbound lanes, according to TDOT officials.

By Travis Dorman of the Knoxville News Sentinel

8:03 a.m. update: All northbound lanes of Alcoa Highway near Cherokee Trail have reopened.

One soutbound lane is open to traffic. The other two lanes are expected to reopen between noon and 12:30 p.m. Thursday, Tennessee Department of Transportation spokesman Mark Nagi said.

Update: All lanes are back open on Alcoa Highway northbound. Reminder, only one lane is currently open to southbound traffic. — Mark Nagi (@MarkNagiTDOT) September 29, 2016

Previous report:

Authorities don't know for sure whether the sinkhole caused the water main break or the water main break caused the sinkhole, but the combination shut down the northbound lanes and all but one southbound lane of Alcoa Highway near Cherokee Trail on Wednesday.

"You have two things happening at basically the same time," Tennessee Department of Transporation spokesman Mark Nagi said. "It turns into a chicken-or-the-egg situation."

Sinkholes have been a common feature of the geology of East Tennessee for millennia, Knoxville Utilities Board spokeswoman Stephanie Midgett said. The sinkholes mainly occur in areas where the rock beneath the land's surface — such as limestone or gypsum — can be easily dissolved by water.

Over time, groundwater erodes the rock, creating caverns and empty spaces until there is no longer enough support for the weight on the surface. Then suddenly, it all collapses.

Construction crews aim to finish repairing the gaping hole, which stretches from the shoulder of Alcoa Highway across two northbound lanes, before morning rush hour, "but if we're not sure the roadway is stable enough then we won't open it," Nagi said.

"We're not going to allow that roadway or any roadway to be open if we feel it presents a danger to the motoring public," he said.

Northbound traffic is being diverted to Gov. John Sevier Highway, and drivers headed south are encouraged to find alternate routes or allow extra time for delays.

TDOT crews worked on filling in the sinkhole throughout the day Wednesday, and less than 50 feet away, a KUB crew worked on repairing the water main.