The hidden landscape under the sea

Stand on a beach and look out. The sea looks flat. But if all the water could be drained away, you would see mountains, valleys, plains and deep gorges, just like on land. These landforms under the sea are called ocean relief or submarine relief. In Indian culture the ocean, Samudra, has always been seen as powerful and sacred.

Now imagine walking out from the shore, step by step (Fig. 1.2):

• Continental shelf. Think of the steps of a ghat going down into the Ganga. The lower steps are under water, but they are still steps. The shelf is the same: it is still the continent, just covered by shallow sea. Sunlight reaches it, so it is full of marine life, and it often holds oil and gas.
• Continental slope. Keep walking, and suddenly the floor drops, like the deep end of a swimming pool. This steep slope is the real edge of the continent.
• Continental rise. At the foot of the slope the ground slopes more gently. Here deep canyons cut into the edge: submarine canyons.
• Abyssal plain. Now you are on the true ocean floor: vast, flat and very deep. Small hills stick up from it: abyssal hills.

Further out are the giants:

• Seamount: an underwater mountain, usually a volcano, that rises from the floor but does not reach the surface. Picture a glass standing at the bottom of a full bucket: it stands on the floor, but stays completely under water.
• Guyot: a seamount whose top was worn flat by waves long ago and which later sank. Its top is flat, like a cone with the point cut off.
• Volcanic island: a volcano that does come up above the water. Picture a spoon standing in a glass of water: it touches the bottom and sticks out at the top.
• Mid-ocean ridge: a huge chain of underwater mountains made by volcanic activity. It is the longest mountain range in the world. Along its middle runs a rift valley, where magma rises from below.
• Trench: the deepest parts of the ocean. The deepest spot on the whole Earth is the Mariana Trench, in the Pacific.

Two surprises. The longest mountain range on Earth is hidden under the sea. And Mauna Kea in Hawai'i, measured from the sea floor, is taller than Mount Everest: most of it is under water.

Why does this matter? The shape of the floor decides where the water is shallow or deep, where fish live, where oil and gas lie, and where ships and undersea cables can safely go.