(WBIR) Engineers have reopened Interstate 40 at the Tennessee and North Carolina line. NCDOT originally said the closure would last until around midnight, but crews wrapped up work and opened the interstate at 6:00 p.m.

Twenty miles of I-40 were closed from Exit 451 in Tennessee to Exit 20 in North Carolina to allow NCDOT crews to clear dangerous rocks from some of the cliffs above the interstate. Thousands of drivers were detoured along I-81 and I-26.

Rockslides are a longtime danger that constantly hangs over the heads of 23,000 vehicles that drive through that portion of I-40 on a daily basis. The beauty of the Appalachians have a long history of crashing down along a 30-mile stretch known as "the gorge" where the interstate hugs the steep cliffs along the Pigeon River.

NCDOT spokesperson Jordan Ashley Baker said Thursday's closure started a week ago when North Carolina crews noticed some sizable rocks along the shoulder near the Harmon Den exit at mile marker 7. Engineers monitored the mountainside and ultimately decided to close the interstate and clear the dangerous rocks on their own terms.

Although the entirety of Thursday's work was done in the westbound lanes next to the hillside, engineers have good reason to close the interstate in both direction.

WBIR has shot a great deal of video through the years that clearly shows falling rocks can change lanes without warning. In 1997, an enormous boulder bounced across four lanes of traffic and over the head of videographer Brian Holt during coverage of a rockslide that shut down I-40 for two months.

In 2009, a massive rockslide crumbled into the interstate and blocked I-40 for six months. NCDOT also installed underground sensors at this site to detect any shifts in mountainside. Solar panels power a cell phone system that will automatically contact engineers when any movement is detected.

The constant threat of rockslides comes with the territory because the road was constructed on the side of the mountain where rock formations naturally slide towards the highway. It is a big problem with small short-term solutions that include rockfall fences, mesh webbing draped on the cliffs, and deep bolts that help secure the hillside.

"We can put up fences and barriers to block some rocks, but really it is a matter of constantly monitoring these areas," said Mark Nagi, TDOT spokesperson. "We want people to let us know if they see rocks along the side of the road so we can keep those areas clear and investigate to make sure there's not a risk of a larger slide."

Drivers who notice rocks along the road can call TDOT at 865-594-2400.