A ship sails away into a burning sunset while the sea beneath it boils with blood and fire. In the lower right, manacled hands and feet break the surface of the water — the dead and the dying, thrown overboard. The sky is a furnace of red and gold, the sea a churn of green and crimson, and the whole picture is beautiful and terrible at once, which is exactly the point: Turner wraps an atrocity in splendour so that the splendour itself becomes an accusation.

Turner painted it in 1840, when he was sixty-five, and exhibited it at the Royal Academy. The subject was the Zong massacre of 1781, when the crew of the slave ship Zong threw 132 enslaved people overboard so that the owners could claim the insurance on them as lost cargo. The story had become widely known in Britain through the abolitionist movement, and Turner — who had already painted the slave trade's horrors in verse — turned it into his most furious picture. It was a political painting made at a moment when Britain had abolished slavery in its colonies only a few years before, and the trade itself was still a living memory.

The technique is pure late Turner: the forms dissolve into light and colour, the ship itself little more than a dark silhouette against the blaze, the sea built from thick, swirling strokes of paint that make the water feel like it is moving. The composition is diagonal and violent — the sunset pulls the eye to the right, the drowning figures pull it down, and the ship escapes to the left, leaving its victims behind. The manacled limbs are painted small, almost lost in the waves, which is the horror of it: you have to look closely to see what the painting is really about, and once you have seen it you cannot unsee it.

The story behind the picture is the Zong case itself, one of the most notorious episodes in the history of the Atlantic slave trade. The enslaved people were thrown overboard alive, on the grounds that they were dying of disease and would be worth more dead — insured — than alive. The insurers refused to pay, the case went to court, and the abolitionist Granville Sharp made it a public scandal. Turner does not paint the courtroom or the ship's deck; he paints the sea afterwards, the moment when the crime is already done and only the water remembers.

The painting's first owner was the critic John Ruskin, who called it Turner's greatest work and kept it as the centrepiece of his collection. It now hangs in the Museum of Fine Arts in Boston. It matters because it is one of the first great paintings to make the slave trade its subject without flinching, and because it asks an uncomfortable question that still lands: how can something so beautiful be about something so cruel — and what does it mean that we are drawn to the beauty first?