People have always wanted to discover new things and new worlds. We have built spaceships to go to other planets and are planning to 13 ______ other galaxies one day. Meanwhile, we know very 14 ______ about our own planet. The oceans, which cover 70 percent of the planet, are an unknown world to us. The Pacific Ocean is the largest one. The 15 ______ point on the Earth is also there. It is 16 ______ Challenger Deep and it was discovered in 1875. The Pacific Ocean 17 ______ from the Arctic Ocean to the coast of Antarctica. But most of its water is in the warm tropics, which 18 ______ it warm. A lot of fish and animals live in the Pacific Ocean. The place is still waiting for researchers to discover its secrets for us.