Earth is largely covered by water: oceans and seas, together with lakes and rivers, make up 71% of the planet,s surface, which explains why, when we look at our planet from space, we see it as a big blue ball. When did oceans and seas form? What is the difference between the sea and the ocean? What organisms live in the marine ecosystem? This book in the What, How, Why series explores various aspects of the seas and oceans, a fundamental resource for life on Earth, through the question-and-answer format.