What caused every flight in the U.S. to be delayed on Wednesday?

Every flight in, to and out of the U.S. was grounded for hours on Wednesday morning. The reason? A single computer system went down. The Notice to Air Missions (NOTAM) system is used nationwide to alert pilots if there are any issues that could affect the safety of their flights. Say a certain runway’s lights aren’t working — NOTAM allows the airport to notify any pilots who will be landing there. It’s a critical piece of aviation safety.

So when it stopped working on Wednesday morning, every plane in the U.S. that wasn’t already in the air was grounded. And it lasted for hours, with the order finally lifting at 9 a.m. Eastern. A reported 7,000 flights were delayed as a result, and another 1,100 were canceled. Though the delay was fortunately short-lived, the incident threw into stark relief the need to improve the digital infrastructure that our country relies on to operate.

“Today’s FAA catastrophic system failure is a clear sign that America’s transportation network desperately needs significant upgrades,” said Geoff Freeman, CEO of the U.S. Travel Association.