It's difficult to definitively say what the most sour substance on earth is because "sourness" is a subjective sensory experience. Different people perceive sourness differently, and there isn't a single, universally accepted measure of sourness.

However, we can look at some contenders:

* Hydrochloric Acid (HCl): This is a very strong acid, and is found in our stomachs to help break down food. It has a very low pH, meaning it's extremely acidic. However, it's not typically considered a "sour" taste, as it's too corrosive to be ingested.

* Citric Acid: Found naturally in citrus fruits, citric acid is a common ingredient in food and drinks. It has a very sour taste, and is often used to add a tangy flavor.

* Tartaric Acid: Found in grapes, tartaric acid is responsible for the sour taste of wine. It's also used as a food additive.

* Acetic Acid: This is the acid that gives vinegar its sour taste. It's also found in many other foods, such as pickles and sauerkraut.

It's worth noting that the sourness of a substance is also influenced by its concentration. A small amount of citric acid might be pleasantly sour, but a large amount could be unpleasantly acidic.

Therefore, while we can identify some substances as being very acidic or having a strong sour taste, it's hard to definitively say which is the "most sour" without taking into account individual perception and concentration.