Alton Towers theme park has been closed while an investigation continues into a crash on their Smiler ride.
Four people were seriously hurt in the collision on Tuesday and were taken by helicopter to hospital.
Officials are trying to work out how it happened and how to keep people safe in the future.
Merlin Entertainment, which runs the park, said: "The park will remain closed until we understand better the cause of this dreadful incident."
It's not known when the park will reopen.
Ricky's been looking into the story.