Quiz of the Day
Oceans & Marine Life
Monday, 8 June 2026
World Oceans Day — deep sea, coral reefs, ocean records, marine creatures
1. World Oceans Day is celebrated on 8 June. What is the deepest known point in the ocean?
- The Tonga Trench
- The Challenger Deep in the Mariana Trench
- The Puerto Rico Trench
- The Java Trench
The Challenger Deep reaches approximately 10,935 metres — deeper than Mount Everest is tall. If you placed Everest at the bottom, there would still be over 2 km of water above its peak. Only 27 people have ever visited the bottom.
2. What is the largest animal to have ever lived on Earth?
- The megalodon shark
- The African elephant
- The blue whale
- The Argentinosaurus dinosaur
Blue whales can reach 30 metres long and weigh 200 tonnes. Their hearts are the size of a small car and can be heard beating from 3 km away. Their tongues alone weigh as much as an elephant. A baby blue whale gains about 90 kg per day.
3. What percentage of the ocean floor has been mapped in detail?
- About 50%
- About 5%
- About 25%
- About 75%
As of 2023, only about 25% of the ocean floor has been mapped to modern standards. We have better maps of the surfaces of Mars, the Moon, and Venus. The Seabed 2030 project aims to map the entire ocean floor by 2030.
4. What causes coral bleaching?
- Chemical pollution from boats
- Stress from warming water temperatures causing corals to expel their symbiotic algae
- Too much sunlight
- Overfishing of reef species
Corals get their vibrant colours from tiny algae called zooxanthellae that live inside them and provide up to 90% of their food through photosynthesis. When water warms just 1-2 degrees C above normal, corals expel the algae, turn white, and can starve to death.
5. How much of the world's oxygen is produced by ocean organisms?
- About 20%
- About 70%
- About 35%
- About 50%
Tiny marine organisms called phytoplankton produce at least 50% of Earth's oxygen through photosynthesis. A single species, Prochlorococcus, is the most abundant photosynthetic organism on the planet and produces about 20% of the oxygen in our atmosphere.
6. What is the Great Pacific Garbage Patch?
- A concentrated zone of floating plastic debris roughly three times the size of France
- A landfill on a Pacific island
- A recycling facility in the ocean
- An island made of trash in the Pacific
Despite its name, it is not a solid mass — most of it consists of microplastics suspended in the water, making it invisible from the surface. It contains an estimated 80,000 tonnes of plastic. The Ocean Cleanup project has been testing systems to remove it since 2018.
7. What is bioluminescence?
- The ability to sense electric fields in water
- Reflection of moonlight off ocean creatures
- The production of light by living organisms
- The process of absorbing sunlight underwater
Up to 76% of deep-sea creatures are bioluminescent, making it the most common form of communication in the ocean. The anglerfish uses a glowing lure to attract prey in total darkness. Some squid even squirt bioluminescent ink instead of black ink.
8. How many oceans are officially recognised?
- 5
- 4
- 7
- 3
The five oceans are the Pacific, Atlantic, Indian, Southern (Antarctic), and Arctic. The Southern Ocean was only officially recognised by the National Geographic Society in 2021. The Pacific alone covers more area than all the land on Earth combined.
9. What is the fastest fish in the ocean?
- Bluefin tuna
- Swordfish
- Great white shark
- Sailfish
Sailfish can reach speeds of up to 110 km/h (68 mph), making them the fastest fish in the sea. They use their distinctive sail-like dorsal fin to herd schools of fish into tight groups, then slash through them with their long bills.
10. What is an octopus's most remarkable cognitive ability?
- Perfect memory
- Problem-solving, including opening jars and using tools
- Telepathic communication
- Echolocation
Octopuses have been filmed unscrewing jar lids from inside, using coconut shells as portable shelters, and even escaping aquariums to hunt in neighbouring tanks before returning. They have three hearts, blue blood, and roughly 500 million neurons — two-thirds of which are in their arms.
11. What is the longest-living marine animal known to science?
- Ocean quahog clam
- Giant tortoise
- Greenland shark
- Bowhead whale
A quahog clam nicknamed 'Ming' was found to be 507 years old when scientists counted its growth rings — it was born in 1499, during the Ming Dynasty. Sadly, the researchers accidentally killed it by opening its shell to determine its age.
12. What are mangrove forests and why are they important for oceans?
- Deep sea vegetation
- Coastal trees that grow in saltwater, protecting shorelines and serving as nurseries for marine life
- Freshwater forests near rivers
- Underwater kelp forests
Mangroves store up to 4 times more carbon per hectare than tropical rainforests. They protect coastlines from storms and tsunamis, and up to 80% of global fish catches depend on mangroves at some point in their life cycle. Yet we have lost over 35% of them since the 1980s.
13. How deep can a sperm whale dive?
- About 3,000 metres
- About 2,000 metres
- About 1,000 metres
- About 500 metres
Sperm whales can hold their breath for up to 90 minutes and dive over 2,000 metres deep to hunt giant squid in total darkness. The pressure at that depth is 200 times atmospheric pressure. Their bodies collapse their lungs and redistribute blood to survive.
14. What is an 'ocean dead zone'?
- A low-oxygen region where most marine life cannot survive
- A part of the ocean too deep for sunlight
- An area with no fish species
- A zone contaminated by nuclear waste
There are now over 700 known dead zones worldwide, up from 49 in the 1960s. They are mainly caused by agricultural fertiliser runoff that feeds algal blooms. When the algae dies, decomposition consumes the oxygen. The Gulf of Mexico dead zone can reach 22,000 square km in summer.
15. What is the Mid-Atlantic Ridge?
- A coral reef in the Atlantic
- A current that flows between Europe and America
- A deep-sea trench
- An underwater mountain range running down the centre of the Atlantic Ocean
At over 16,000 km long, it is the longest mountain range on Earth — dwarfing the Andes. It marks where tectonic plates are pulling apart, creating new ocean floor. Iceland sits directly on it and is one of the few places where the ridge is visible above sea level.