If you’ve ever entertained the possibility of holding your breath through the Eisenhower Tunnel, you know how long it is.

And a lot of impressive engineering goes into this Interstate 70 landmark beyond the fact – you know – that there’s a highway under a freaking mountain.

One feature? Lots of sprinklers in case there’s a fire inside the tunnel.

Twenty percent of these sprinklers are tested each year, and that’s why the tunnel will be closed overnight for two days next week.

The sprinklers were first installed in 2015 as part of a $20 million fire suppression project. Each year, some of the valves are “exercised” as part of the testing procedure, which ensures none of the sprinklers are corroded and will work in case an emergency happens.

These tests will happen through all 30 years of the expected life of the system, the Colorado Department of Transportation said.

The highway needs to close during the sprinkler tests because they are spraying water over both lanes. Crews will take advantage of this time to also perform maintenance and inspection work on lightning, the road and other structural elements in the tunnel.

Here’s when the tunnel will be closed and traffic will be detoured to Loveland Pass:

July 9: 10 p.m. to 6 a.m. (westbound)
July 10: 10 p.m. to 6 a.m. (eastbound)

The left lane of the westbound tunnel will be closed on July 11 from 10 p.m. to 6 a.m., and the left lane of the eastbound tunnel will close from July 12 from 10 p.m. to 6 a.m.