Cannonball! This participant at a Tough Mudder event in New Jersey splashes into the Quagmire, a 2.4 meter (8 foot)-deep mud pit. It was just one of the many muddy obstacles competitors had to run, crawl, and swim through during the 16 kilometer (10 mile) course.

To create the huge amount of mud for one of the events, workers simply mixed together vast quantities of soil and water, says Marina Kreatsoulas, who works for Tough Mudder. The clinging force of surface tension causes the water droplets to stick to the soil particles. The same force also causes the droplets to stick to each other, forming microscopic water bridges between specks of dirt. These bridges act like glue to hold the mud together. It takes about 3,800 liters (1,000 gallons) of water to make the Quagmire a “genuinely sticky situation,” says Kreatsoulas.

Trying to move through the many gloppy obstacles is partly what makes Tough Mudder so tough. As contestants run the course, their shoes sink into the mud, squeezing out the air between their shoes and the gooey ground. That causes the air pressure under their feet to become lower than the air pressure above. This difference in the force of air molecules creates suction, making it difficult for racers to pull their feet from the goop.

Competitors finish each Tough Mudder event covered in grime. But getting super messy is all part of the fun. Each year, more than 100 of the events take place in 11 countries around the world, including Mini Mudders for kids. If you want to participate, be prepared to get dirty.