Those six words were the last radio transmission from the cockpit of Malaysia Airlines Flight 370, less than an hour after the aircraft took off late at night from Kuala Lumpur to Beijing on March 8, 2014. Minutes later, the plane disappeared from air-traffic control radar screens.

The huge Boeing Co. 777 jet, almost as long as a Manhattan city block and taller than a five-story building, had somehow managed to make itself invisible in the clear night sky. There were 239 people on board.

Ensuing search operations combed through some of the deepest ocean floors in the inhospitable southern Indian Ocean, hundreds of miles off Australia’s western seaboard, and found no trace of the main fuselage or any passengers and crew. Of the 3 million components in the 777, just a few fragments washed ashore years later on the east African coast.

With no mayday call, no known flight path and no wreckage, MH370 remains modern aviation’s biggest mystery. And while investigators had very little to go on, they were clear on one thing: A plane must never go missing like this again.