The curious case of Alonso's mysterious testing accident

It is February 22nd, 2015 and Formula 1 pre-season testing is at Spanish Grand Prix venue Barcelona.

After a troubled opening test at Jerez, McLaren-Honda is finally hoping to put some heavy mileage on its new power unit and allow World Champions Jenson Button and Fernando Alonso vital time behind the wheel.

But all is still not going to plan as Alonso has been forced to miss the first couple of hours of the day's running as Honda tried to configure the power unit with a new MGU-K seal design following trouble.

Alonso finally gets out on track, climbing to fifth place, but on his 21st lap of the day, the two-time champion suffered a mysterious crash that has sparked ongoing debate to this day, with still no conclusive answer.

What actually happened to Alonso?

Rounding the long right-hander of Turn 3, Alonso's McLaren suddenly veered to the right and crashed into the bare concrete wall.

It was something of a glancing blow at nearly 100mph as the car straightened whilst on the grass - which we will come back to later.

Sebastian Vettel had just exited the pits for Ferrari and was the nearest car to the McLaren - and described the car's movement as "strange."

Usually after such an innocuous-looking accident, Alonso would have jumped out the car, but when he did not, medical extraction crews went to his assistance.

He was promptly air-lifted to hospital where he spent three days and was diagnosed with concussion after the heavy impact.

For fear of a secondary impact in the season-opening Australian Grand Prix, he sat out the race, returning in round two in Malaysia - which Vettel would win as Alonso recorded the first DNF of his second McLaren spell.

What are the theories behind it?

McLaren claimed that Alonso had simply been caught out by strong, unpredictable gusty winds which gained some credence when Carlos Sainz suffered a similar incident later that afternoon at Turn 3.

When he finally addressed the crash in Malaysia, Alonso refuted that theory - and blamed the car.

"It was clear there was a problem in the car, but it was not found in the data," he said.

"There is not a clear answer. We definitely had a steering problem in Turn 3, and it locked to the right.

"I downshifted from fifth to third. Unfortunately, in the data, we [are still] missing some parts.

"Whatever part was the problem, perhaps we'll never find out."

That was given short-shrift by McLaren, but what is known is that he was knocked briefly unconscious after the impact.

As the car had straightened seconds before the first impact, the suspension did not break on contact with the wall as the car went in largely parallel.

As the energy of the impact did not break the suspension, it had to go somewhere - and it did - through the unwitting Alonso.

After he came around, it was claimed that he woke up believing he was still a Ferrari driver or that it was 1995 and he was a go-karter hoping to make it to F1.

That was also rubbished by Alonso in that Malaysia press conference

"I didn't wake up in '95, speaking Italian, or anything like that. I remember the accident and I remember everything the following day," he said.

One final theory is that Alonso was electrocuted by his car, with Honda struggling to understand the new complex energy recovery systems or that he had passed out due to toxic fumes from the batteries.

Former F1 racer Fabrizio Barbazza claimed that, from his sources, Alonso had suffered a 600-watt hit from the car.

The crash was captured on CCTV, but the footage has never been made public.