The most common type of planet in our Galaxy — rocky worlds around other stars, or exoplanets, shrouded in the simplest gas hydrogen. Their innards remained a mystery until scientists recreated the conditions of the interior in the lab. It turns out that inside, such a planet works like a giant pressure cooker: at heat above 4000°C and monstrous pressure, hydrogen penetrates molten rock, stealing oxygen from iron oxides. Water is born, and the freed iron gathers into heavy droplets, like grains in a boiling broth. In one cooking cycle, a mass of water comparable to dozens of Earth's oceans accumulates, and simultaneously heat is released that keeps the planet warm from within for a long time. This underground kitchen rewrites the history of water in the Universe: oceans can appear not only from comet impacts, but also directly from the primordial gas that enveloped the newborn world. And if our young Earth once wore a hydrogen coat, its own global ocean could have been brewed in the same rocky cauldron.