The Smiler: Facts about the Alton Towers rollercoaster

West Midlands Ambulance Service says four teenagers have been seriously injured after two carriages on The Smiler ride at Alton Towers collided.

Emergency crews had to build a platform to help reach people stuck on the ride.

It isn't the first time The Smiler has been hit with problems since opening two years ago. Here's what we know.

• It has the most inversions - or loops - of any rollercoaster in the world but even before today's crash the ride had a complicated history.

• Its original opening was delayed because of what Alton Towers resort described as "teething problems".

• In the July of its opening year, the ride shut briefly after a bolt was seen falling from the tracks.

• In November 2013 wheel parts came off a carriage at one of the highest points.

Kirsty Wild was in the queue for the Smiler ride.

The ride has plunging drops of up to 30m and extreme turns.

It lasts almost three minutes and is one of the longest of any theme park worldwide.

Compared to other rollercoasters in the park, it's huge, with a track length three times that of the Oblivion, a former lead attraction at the resort.

The frame took more than eight months to build and suffered delays because of poor weather during construction.

The Smiler is the equivalent of 10 football pitches of ride track and dominates the X-sector area of the Staffordshire theme park.

The rollercoaster was the biggest ever investment for a ride at the resort, costing £18m to build, and celebrities were invited to open it back in 2013.

Helen Critchley was in the area of Alton Towers when it happened.

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