The title of "oldest known living organism" is a bit tricky, as it depends on what we consider an organism.

Here are two contenders:

* Pando, a clonal colony of quaking aspen trees: Located in Utah, Pando is estimated to be around 80,000 years old. It consists of a single, massive root system that sends up thousands of genetically identical trees, forming a giant, interconnected organism.

* A colony of *Posidonia oceanica* seagrass: Found in the Mediterranean Sea, this seagrass colony is estimated to be around 100,000 years old. Like Pando, it's a clonal organism, meaning it spreads vegetatively and is essentially one giant individual.

It's important to note that there is some debate on the exact age of both Pando and the *Posidonia* colony, and other contenders for the oldest organism exist. Ultimately, the question of the oldest organism is complex and subject to ongoing research.