Signs of reactivation began to occur in 1961 in the form of microseisms; previously, several specialists warned that the Irazú Volcano could explode at any time.

It was a powerful explosion that surprised a large part of the country just when John F. Kennedy was making a state visit to Costa Rica in San José,[2] a thin layer of ash fell on everyone, including Kennedy.

On December 9, 1963, the most catastrophic event of the eruption, known in Spanish as La Noche Gris (The Grey Night), was where a lahar slid down the Reventazon River, destroying everything in its path, including trees and some buildings in rural areas, killing some livestock such as cows and horses, and huge rocks that came down the river very quickly, and when the stones collided, sparks could be seen, provoked of the size of the stones, accompanied by a powerful noise.

The river overflowed, causing even more damage, especially when it arrived to Cartago, where he took with him a lot of houses and roads cumpletly covered in mud and ash.

Many emergency services arrived at the scene to try to rescue people stuck in the destroyed houses.