The longest mountain range on Earth is almost entirely underwater

The global mid-ocean ridge system winds for roughly 65,000 km around the planet, longer than the Andes, Rockies, and Himalayas put together, yet more than 90% of it lies submerged and out of sight. It marks the seam where tectonic plates are spreading apart, with magma rising through the gap to build fresh seafloor on either side.

— NOAA National Ocean Service, What is the longest mountain range on earth? — NOAA Ocean Facts