Corals are small marine animals. The “hard” type of coral grows with the help of algae, which gives it color and nutrients that help it form a sturdy, stony skeleton. Reefs, which form through the accumulation of hard coral, are a vital ecosystem, supporting an estimated 25 percent of ocean species. Corals face threats including ocean acidification and rising temperatures, and National Geographic Explorers are at the forefront of studying and protecting them.