The impact released as much energy as 100 million nuclear bombs, gouging a 200-kilometer-wide, 20-kilometer-deep scar in Earth's crust and unleashing monstrous earthquakes, tsunamis and firestorms.
Alvarez and his colleagues discovered a layer of debris laid down in 66-million-year-old rocks around the world that was curiously enriched with elements such as iridium, which is rare in Earth's crust but abundant in asteroids and comets.
This planetary cataclysm remained shrouded in mystery until physicist Walter Alvarez pieced together its outline in the 1970s and 1980s.
For decades, scientists have debated finer details: Was the impactor an asteroid instead of a comet and if so, what type? Where in space had it come from?
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