Why does starship explode when it hits the water?

On its test flights Starship and Super Heavy have splashed down in the Gulf of Mexico and the Indian Ocean on purpose, because SpaceX does not yet attempt to catch every vehicle and wants to prove the landing sequence without risking the launch site. The sequence ends with the vehicle hovering at zero speed above the water, exactly as it would above the tower, and then dropping in. A 50 m stainless-steel stage standing upright on the sea is unstable: it topples, the thin tank walls are not designed for side loads, and the structure breaks apart.

Both stages carry residual propellant on arrival: the landing burn cannot be flown with empty tanks, and margins are kept for engine relights. When the tanks rupture, liquid methane and liquid oxygen spill and vaporise in an instant. The mixture needs only a small ignition source, such as a hot engine or a static discharge, to produce the fireball seen in SpaceX's footage. The same physics applies to the booster, which on several flights has tipped over and exploded seconds after a successful soft landing at sea.

Not every explosion has been a soft landing gone as planned. On some flights the ship was lost during re-entry or before reaching the target zone, and on the first integrated flight in April 2023 the flight termination system was activated when the stack lost control. Those events are investigated as mishaps under FAA oversight before the next launch is licensed. The distinction matters: a vehicle that reaches the water under control has finished its test; one that breaks up beforehand has not.