For centuries humans believed the ocean was so vast that it was impossible to do it measurable harm. But we now know human activities can destroy critical marine habitats, dangerously pollute seawater and make sea environments more acidic. Overharvesting has disrupted food chains and directly pushed many ocean species into the critically endangered category—and has driven some animals, including Steller's sea cow, into total extinction. This past March the smooth handfish, Sympterichthys unipennis, officially became the first modern-day marine fish to be declared extinct.