How it unfolded
Twenty-four-year-old Eddy Merckx was already overall leader by nearly eight minutes, with no need to attack on this 214 km stage over the Peyresourde, Aspin, Tourmalet and Aubisque. He went anyway, riding away alone from the top of the Tourmalet with 140 km still to race. His team car repeatedly told him to slow down, but he crossed the Aubisque and rode 60 flat kilometres alone all the way to Mourenx. The chase group finished 7 minutes 56 seconds later, stretching his overall lead to nearly 16 minutes. He went on to win his first Tour by 17 minutes 54 seconds over runner-up Roger Pingeon, and swept the points and mountains classifications too, the only rider ever to hold all three major jerseys at once at a single Tour. Asked why he had attacked, he said there was no particular reason.
Why it still matters
The nickname "The Cannibal" was earned on this day. Leaving nothing for anyone else even in a race already won became the standard the entire peloton was held to for the next decade, and it explains how Merckx's career total of 525 wins was even possible. No rider has held all three major jerseys at a single Tour since.