They were wondering why the mud had a certain scent to it.
More than 1,000 runners have fallen ill after competing in a French mud race in which it was suspected animal faeces had become mixed with the mud and dirt.
Held on June 20, pictures taken at The Mud Day race showed participants crawling through puddles and over obstacles coated in dirt.
Initially only 30 people complained of diarrhea and vomiting, but in the days following this figure skyrocketed to more than 1,000.
However, it is thought the illness may have been caused by animal manure that had mixed with the mud, which some participants claimed they could smell on the day.
The course itself stretches across 8miles and runners must cross electrified barbed wire, climb through muddy tunnels and over dunes.
Read more: Daily Mail