Oceans cover more than two-thirds of the surface of the planet - yet they are still not fully understood.
The world's oceans play an important role in the Earth's climate system, and millions of people depend on them directly for sustenance. People also traverse the oceans, and use them to transport goods. Yet the world's oceans are in trouble, as warming and acidification take their toll. They've also become a garbage dump for the planet, with consequences for marine life.
Part of the Mesoamerican Reef has just come off the list of endangered UNESCO heritage. In Mexico, mass tourism is to fund a first-of-its-kind insurance plan to restore its reefs after hurricanes. But challenges remain.