Input (grade 10): A majority of the planet's solid surface is abyssal plain, at depths between 4000 and 5500 m (13,000 to 18,000 ft) below the surface of the oceans.
Output (grade 6): Most of the sea-bed is between 4000 and 5500 m (13,000 to 18,000 ft) below the surface of the ocean.