SpaceX has a major announcement for everyone who has been trying to understand why a rocket exploded on its launch pad in Florida last year: they finally have a cause. That’s a big deal because it the anomaly was, according to SpaceX founder Elon Musk, “the most difficult and complex failure we have ever had in 14 years.”

Here’s what SpaceX believes happened. There are three aluminum vessels in the rocket that contain super-chilled liquid helium inside of a main tank that holds the liquid oxygen the rocket uses for combustion. The vessels release helium to replace oxygen when it is consumed, keeping the pressure of the fuel tank stable.

There is a shrink-wrapped carbon composite on the outside of these vessels, which sometimes develop tiny wrinkles between it and the lining made out of aluminum. Oxygen can seep into these wrinkles. SpaceX thinks that the helium pumped into the vessels was too cold at the time, causing the oxygen in these wrinkles to become frozen and turn into a solid, which then reacted with the composite exterior somehow, and that punctured the vessel and caused the huge explosion.

The SpaceX statement reads: “Each stage of Falcon 9 uses COPVs to store cold helium which is used to maintain tank pressure, and each COPV consists of an aluminum inner liner with a carbon overwrap. The recovered COPVs showed buckles in their liners. Although buckles were not shown to burst a COPV on their own, investigators concluded that super chilled LOX can pool in these buckles under the overwrap. When pressurized, oxygen pooled in this buckle can become trapped; in turn, breaking fibers or friction can ignite the oxygen in the overwrap, causing the COPV to fail. In addition, investigators determined that the loading temperature of the helium was cold enough to create solid oxygen (SOX), which exacerbates the possibility of oxygen becoming trapped as well as the likelihood of friction ignition.”