It is an ancient ceremony of mysterious origin. Every May, farmers in La Esperanza, Guerrero, Mexico, plant crops. The seeds need water to grow, so the villagers perform rituals that encourage rain.
This means collecting human blood in buckets and plowing it into the ground. The blood is the result of fistfights that the women have for the occasion. Rodrigo Cruz of Vice reports:
The villagers quickly began arriving in their dozens, forming a human perimeter while they waited for their out-of-town adversaries to reach the makeshift battlefield. Once the neighboring communities began to show up, women immediately started seeking out opponents and challenging them to a fight. The mothers and grandmothers, many of whom had been warriors in the past, got busy urging the younger generation of girls to get out in the ring, to split some skulls and spill some blood.
Once they were set on an opponent, the women faced each other, tying their hair up and removing rings or anything else that might get in their way. Then, often with their eyes closed, they threw the first punch. Occasionally they'd grab a handful of dirt to dry the sweat on their hands, or ask for a time-out to clean their bloodied noses, but they'd always carry on relentlessly tearing into each other, with cries of support resounding from all sides of the fight pit.
-via Oddity Central (warning: auto-start video)