Answer the following questions.

Questions 16-20 are based on the following text.

    The sea is absolutely vast. Salty seawater covers about two-thirds of our planet, so there is more sea than land. The sea includes five oceans, which are the Southern, Atlantic, Pacific, Arctic, and Indian.

    By far the biggest ocean in the world is the vast Pacific. It covers a third of the Earth. At its widest point, between Panama and Malaysia, the Pacific stretches almost halfway around the world.

     The sea's saltiness comes from salt. It is the same salt you sprinkle on your food. The rain washes the salt on to rocks and rivers carry it to the sea.

     The seabed is not flat. There are volcanoes, mountains, and valleys on the seabed. The Mid-Atlantic Ridge is a long line of underwater mountains in the Atlantic Ocean. Steep underwater valleys are known as trenches.

"By far the biggest ocean in the world is the vast Pacific."

The underlined word means ....