The ocean holds 50 times more carbon than the entire sky above us, and right now it's pulling about 11–15 billion tonnes of carbon out of the air every year without us doing a thing. So the question of this trend is simple — if the ocean is already a giant sponge that's good at absorbing carbon, can we "squeeze the sponge to make room" and get it to absorb a little more? This is the story of the climate fix with the biggest potential and the lowest cost in theory — but also the hardest to prove and the most hotly debated in the field.