Here's why:

* Temperature: Tundra biomes are characterized by extremely low temperatures for most of the year, often below freezing. Average annual temperatures are below 5°C (41°F).

* Location: Found in the Arctic and on high mountaintops, the tundra experiences long, dark winters and short, cool summers.

* Permafrost: A defining feature of tundra is permafrost, a layer of permanently frozen ground that limits plant growth and water drainage.

While other biomes like the polar ice caps and the deep ocean are also very cold, the tundra is considered a distinct biome with a unique ecosystem and is the coolest biome found on Earth's land surface.