The ocean covers 71%! The Pacific Ocean alone covers about 30% of Earth's surface. The ocean contains 97% of Earth's water (the rest is freshwater in glaciers, ice caps, lakes, rivers, and groundwater). The ocean produces over 50% of the oxygen we breathe (via phytoplankton). It also absorbs about 30% of human-produced CO₂, helping to mitigate climate change (but causing ocean acidification). The ocean is largely unexplored – we have mapped less than 20% of the ocean floor. So the answer is 71%.

The Pacific Ocean is the largest! It is so large that all of the world's continents could fit inside it. The Pacific Ocean was named by Ferdinand Magellan (1519-1522), who called it "Mar Pacífico" (peaceful sea) because of its calm waters. The Mariana Trench (Challenger Deep) is the deepest point, at about 36,070 feet (10,994 meters). The Pacific is also home to the Great Barrier Reef (Australia) and many island nations (Polynesia, Micronesia, Melanesia). So the answer is the Pacific Ocean.

The sunlight zone has the most life! It contains 90% of all marine life. The twilight zone (200-1,000 m) receives very little light; animals there often have large eyes or bioluminescence. The midnight zone (1,000-4,000 m) is pitch black. The abyssal zone (4,000-6,000 m) covers much of the deep ocean floor. The hadal zone (6,000-11,000 m) includes the deepest trenches. So the sunlight zone is the answer.

Coral bleaching is caused by warm water! Climate change (ocean warming) is the main cause. Other causes: pollution, ocean acidification, and low tides. Bleached corals can recover if conditions improve quickly. The 1998, 2010, and 2014-2017 bleaching events caused widespread damage. The Great Barrier Reef has experienced mass bleaching in recent years. So coral bleaching is caused by warm water (temperature stress).

Phytoplankton are producers! They are microscopic plants (algae). Zooplankton are tiny animals that eat phytoplankton. Krill are small crustaceans that eat phytoplankton and are eaten by whales, seals, and penguins. Without phytoplankton, the ocean food web would collapse. Phytoplankton blooms can be seen from space. So phytoplankton are producers.

Chemosynthesis is the answer! Bacteria (chemosynthetic) convert hydrogen sulfide into organic matter. Tube worms have symbiotic bacteria that perform chemosynthesis. Hydrothermal vents are found along mid-ocean ridges. The discovery of chemosynthetic ecosystems revolutionized our understanding of life. So chemosynthesis is the process.

The blue whale is the largest! Blue whales eat up to 4 tons of krill per day. They are endangered due to historic whaling. Their heart is the size of a small car. Their tongue weighs as much as an elephant. So the blue whale is the answer.

CO₂ causes ocean acidification! Ocean acidification is sometimes called "climate change's evil twin." It affects pteropods (sea butterflies), which are a key food source for salmon, herring, and other fish. Coral reefs are also vulnerable. Solutions include reducing CO₂ emissions. So carbon dioxide is the answer.

The Mariana Trench is the deepest! It is named after the Mariana Islands. The trench is where the Pacific Plate subducts beneath the Philippine Plate. Deep-sea creatures (amphipods, snailfish) have been found there. The pressure is intense (about 15,750 psi). So the Mariana Trench is the answer.

MPAs are protected areas! They are like national parks underwater. Some MPAs allow limited fishing; others are fully protected (no-take zones). The largest MPA is the Papahānaumokuākea Marine National Monument in Hawaii (over 580,000 square miles). The goal of the "30x30" initiative is to protect 30% of the ocean by 2030. So MPAs are protected areas.