It's tough to say definitively what the smelliest thing on Earth is, as "smell" is subjective and depends on individual sensitivity. However, some contenders for the title include:

* Durian: This tropical fruit is notorious for its pungent, overpowering odor, often described as a mix of sulfur, sewage, and gym socks.

* Stinky Tofu: This fermented tofu dish from Taiwan is known for its strong, ammonia-like smell.

* Casu marzu: This Sardinian cheese is intentionally infested with fly larvae, which break down the cheese and create a very pungent, ammonia-like smell.

* Surströmming: This fermented Baltic herring is known for its strong, putrid odor. It's often described as smelling like rotten fish, sewage, and even death.

* Human feces: This is undoubtedly the most common source of strong, unpleasant odor.

It's worth noting that what one person finds "smelly" another might find "pleasant" (like the smell of durian, which some people actually enjoy). Ultimately, the "smelliest thing on Earth" is a matter of individual perception and cultural context.