The chain of mountains that extend into Earth's oceans are called mid-ocean ridges.

Here's a breakdown:

* Mid-ocean ridges are underwater mountain ranges that form at divergent plate boundaries, where tectonic plates move apart.

* Volcanic activity is a key feature. As the plates separate, magma rises from the mantle and erupts, creating new oceanic crust.

* Longest mountain range on Earth: Mid-ocean ridges are the longest mountain range on Earth, stretching over 40,000 miles (64,000 km).

* Examples: The Mid-Atlantic Ridge, the East Pacific Rise, and the Indian Ocean Ridge are examples of mid-ocean ridges.

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