What's the worst thing you've ever smelled?

For me there are two strong ones competing: Hákarl, basically fermented rotting shark – this smells like a Serbian railway toilet during the war, when it wasn't cleaned for years, but used regularly. It tastes the same. The other was a dog carcass, that was left on the roadside on a nice, hot summer day, and the 'not my job' mentality left it there slowly decomposing over roughly two-three weeks.