NASA’s first crewed moon mission in decades was delayed as its engineers grapple with an all-too-familiar rocket problem: hydrogen leaks. Fixing the issue is even trickier than it may seem.

Forty years after the Challenger disaster, NPR explores the engineers' last-minute efforts to stop the launch, their decades of guilt and the vital lessons that remain critical for NASA today.

Beneath NASA’s launch pads lies a forgotten emergency bunker designed for catastrophe. Astronauts and ground crews could ...