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.
Officials say one of the carriages with 16 people on it, crashed into another, empty carriage on a low section of the track.
Nick Varney, head of Merlin Entertainments, thanked the emergency teams and apologised to those affected and their families.
He said: "This has been a terrible incident and a devastating day for everyone here.
"We have a very strong record of safe operation of our rides here at Alton Towers and it is our priority."
Rollercoaster crashes like this are incredibly rare.
The International Association of Amusement Parks and Attractions say that the chance of being injured on an amusement park ride is one in 24 million.