Magnesium dimer (Mg2) is a fragile molecule consisting of two weakly interacting atoms held together by the laws of quantum mechanics. It has recently emerged as a potential probe for understanding fundamental phenomena at the intersection of chemistry and ultracold physics, but its use has been thwarted by a half-century-old enigma — five high-lying vibrational states that hold the key to understanding how the magnesium atoms interact but have eluded detection for 50 years.