Thousands of athletes turned out at a military base near Madrid in sweltering heat on Saturday to compete in a very dirty obstacle course event.

They ran in mud, treaded knee-deep in mud, even swam in muddy waters.

The Mud Day is an extreme and unusual event that test physical and mental abilities. The challenge runs on a course of nearly 13 kilometers with over 20 obstacles, including water, electricity, ice, and mud. Athletes can participate alone, but to pass the most demanding obstacles, many need to count on a team.

The Mud Day race was held under the slogan "running is not enough" and drew many people eager to navigate a slippery course that included tunnels, dunes and even barbed electric wire.

Created in 2013, the Mud Day race has held previous competitions in France. Upcoming stops in Spain include Valencia, Seville and Barcelona.