Earth’s inner core may be filled with a weird substance that is neither solid nor liquid, according to a new study.

For more than half a century, scientists believed that Earth‘s deepest recesses consist of a molten outer core surrounding a densely compressed ball of solid iron alloy. But new research, published Feb. 9 in the journal Nature, offers a rare insight into the inner structure of the planet — and it’s far weirder than previously thought.

New computer simulations suggest that Earth’s hot and highly pressurized inner core could exist in a “superionic state” — a whirling mix of hydrogen, oxygen and carbon molecules, continuously sloshing through a grid-like lattice of iron.