An increasing awareness of our effect on the ocean is slowly seeping into the public agenda, dragging behind it a conversation that is decades overdue. But this conversation faces a massive obstacle. It's almost impossible to discuss what to do about something changing if you don't initially know how it works. If a doctor tells a patient that they have a problem with their kidneys, the patient probably already has at least a vague idea about where their kidneys are and what they're up to. They learned about that part of their own personal life-support system at school. But that's not the case for the oceans. When we see a news story about the long-term decline in the numbers of krill in the Southern Ocean, it sounds generally like a bad thing. But there's far more to it than the risk of whales going hungry. Krill are a part of the ocean engine. We need to understand at least some of the context before we can discuss the change and take appropriate action.