NEW YORK (AP) — The world's largest aircraft fleet was grounded for hours by a cascading outage in a government system that delayed or cancelled thousands of flights across the U.S. on Wednesday.

The White House initially said that there was no evidence of a cyberattack behind the outage that ruined travel plans for millions of passengers. President Joe Biden said Wednesday morning that he's directed the Department of Transportation to investigate.

Whatever the cause, the outage revealed how dependent the world's largest economy is on air travel, and how dependent air travel is on an antiquated computer system called the Notice to Air Missions System, or NOTAM.

Before commencing a flight, pilots are required to consult NOTAMs, which list potential adverse impacts on flights, from runway construction to the potential for icing. The system used to be telephone-based, with pilots calling dedicated flight service stations for the information, but has moved online.