The flagship race runs roughly 165 kilometers from one end of Réunion to the other. The distance alone would be punishing on flat ground, but Réunion has almost none. The course piles up around 10,000 meters of climbing - the rough equivalent of starting at sea level and ascending the full height of Mount Everest, then doing it again partway. Elite runners finish in just over a day. Most ordinary competitors are out on the trails for two nights, walking when they can no longer run, headlamps bobbing through forests and along ramparts where a wrong step ends in a ravine. There is a generous time limit precisely because finishing at all is the achievement.

What makes the route legendary is geography. Réunion was built by two volcanoes, and the older one collapsed into three vast amphitheater valleys - the cirques of Cilaos, Mafate, and Salazie. The Diagonale threads through all three. Runners cross the lava fields below Piton de la Fournaise, one of the world's most active volcanoes, then plunge into Mafate, a cirque so steep and roadless that the villages inside it are still supplied by helicopter and mule. To run the race is to read the island's entire geological story with your legs: volcanic rock underfoot, erosion towering overhead, the Indian Ocean glittering far below whenever the clouds tear open.

The event began modestly in 1989 as the Marche des Cimes, a march across the heights linking Saint-Denis to Saint-Philippe, with several hundred participants. It grew into one of the most coveted ultramarathons on Earth, drawing runners from dozens of countries and a roadside crowd of Réunionnais who turn out through the night to cheer strangers up the climbs. Today three races share the trails at once: the full Diagonale, the shorter Trail de Bourbon, and the Mascareignes.

The danger is not theatrical. The race has claimed lives. In 2002, Gérard Bordage died on the descent of the Kerveguen slope, and the Dutch runner Guus Smit died on the rampart of the Roche Écrite the same year; in 2012, Thierry Delaprez fell into a ravine at the Col de Fourche. These deaths are remembered soberly on the island, a reminder that the cirques that look so beautiful from a plane are genuinely unforgiving terrain. Runners do not court that risk lightly. They train for years, and they treat the mountains with respect, because Réunion grants its diagonal to almost no one cheaply.