On 1 May 1994, Ayrton Senna was killed at the San Marino Grand Prix when his car left the road at Imola's high-speed Tamburello curve and struck a concrete wall.
Although the true cause of the accident remains a mystery to this very day, what cannot be denied is that the three-time world champion's demise was the single most significant moment in the history of Formula One.
Senna, in life and in death, revolutionised the art of grand prix racing.
His brave, bold and often ruthless approach on track had never been seen before he made his debut in the mid-1980s, an era when drivers were loath to play dare with another car due to the very real risk of serious injury.
If Senna's conduct on the circuit encouraged future generations of drivers to compete with similar levels of aggression and assertion, his death—only 24 hours after Roland Ratzenberger's fatal crash at the same venue—allowed them to do so in secure environments.
Since that afternoon, safety measures have been improved beyond recognition with drivers now compelled to wear, for instance, Head and Neck Support devices and modern-day circuits featuring large run-off areas, lined with high-tech crash barriers to ensure most accidents are minor ones.
Twenty-one years on from Imola 1994, here's how a selection of F1 teams and drivers marked the anniversary of Senna's passing on social media.
In what was one of the most successful periods in Formula One history, Senna took 35 grand prix wins and claimed each of his three world championships with McLaren between 1988 and 1993.
It was at the Woking-based team where the Brazilian became a true star of the sport, renowned for his blistering pace and igniting the rivalry with Alain Prost.
It was unsurprising, therefore, that McLaren were leading the tributes to Senna with this:
Senna had strong links to Honda throughout his career. His affiliation with the Japanese manufacturer began when Honda replaced Renault as Lotus' engine supplier in 1987, and the Brazilian's relationship with the company played a role in his signing for McLaren the following year.
Honda powered Senna to each of his world titles, and Ayrton returned the favour by aiding the development of their NSX road car, with the Brazilian's efforts to leave McLaren accelerating when Honda withdrew from F1 at the end of 1992.
Honda have returned to F1 for 2015, re-establishing their all-conquering alliance with McLaren, and the company have marked the 21st anniversary of Senna's death by posting this on their official Twitter account:
Remarkably, the modern-day Lotus team carries two links to the early days of Senna's F1 career.
The Enstone-based outfit found on the grid these days is the current incarnation of the team that started life as Toleman, who offered Senna his grand prix debut in 1984, while Senna drove for the original Team Lotus between '85 and '87.
Senna confirmed his status as a star of the future during his time with Toleman and Lotus, and now, as two teams united, Lotus offered this tribute:
Senna drove his first and last laps in Formula One behind the wheel of a Williams car. The Brazilian was let loose in reigning world champion Keke Rosberg's FW08C at Donington Park in mid-1983, and a decade later, the Grove-based team lured Senna from McLaren for '94.
Senna, beaten by Williams in the 1992 and 1993 seasons, only competed in three grand prix weekends for the team before his death, and the team have ran with a tribute on their cars ever since that fateful weekend.
"If you think I'm good, wait until you see my nephew," Ayrton once said of Bruno Senna, according to BBC Sport's Andrew Benson.
Any chance Bruno had of emulating Ayrton's success, however, ended following the loss of his uncle and then his father in a 1996 motorcycle accident, after which his mother, per ESPN, banned him from racing for a decade.
The experience he missed out on over those years meant Bruno was always unlikely to emerge as a top-line F1 driver. But he has followed in Ayrton's footsteps to some extent by testing for Honda in 2008, driving for Lotus-Renault in 2011, completing a full season for Williams-Renault the following year and joining McLaren's GT program for this season.
And Bruno has offered a glimpse of how mischievous his uncle could be away from the serious, intense environment of the circuit:
Rubens Barrichello suffered the first major accident of the Imola 1994 weekend in Friday qualifying when his car launched into the air and smashed into the tyre wall before landing upside down.
The crash seemed far more violent than the accidents which claimed the lives of Ratzenberger and Senna, but the Jordan driver emerged from the wreckage with relatively minor injuries.
Barrichello, driving for Ferrari, would go on to become the first Brazilian driver since Senna to win a race at the 2000 German GP, an occasion which moved him to tears.
Just like Senna in the mid-1980s, Romain Grosjean is spearheading Lotus' assault this season.
The Frenchman was just eight—and presumably just getting to grips with a kart—at the time of the Brazilian's death, but the fact he could comprehend the impact of Senna's demise at that age underlines just how significant an event Imola 1994 was in the history of motor racing.
Mark Blundell, like many of his fellow competitors, no doubt felt fortunate to leave Imola unscathed in 1994. Driving for Ligier, the Briton qualified 12th and went on to finish ninth, two laps behind race winner Michael Schumacher.
But for once, the result paled into insignificance.
The Brazilian's influence on F1 is evident over the course of every grand prix weekend, every time a driver slings their car down the inside of another car and every time a driver plucks out their steering wheel, clambers out of the cockpit and walks back to the pits after a trip to the barriers.
Over two decades on, the sport remembers Senna, the greatest there ever was, as fondly as ever.