For nearly four centuries, Jupiter's moon Europa was sensibly considered a frozen, geologically dead world. Astronomers saw a bright point of light, confirmed its water ice, and logically concluded it was an inert relic of the outer solar system. This reasonable assumption held until a magnetometer aboard the Galileo spacecraft detected the electrical whisper of a hidden, global ocean.

This episode explores how our understanding of Europa shifted from a static ice ball to an active ocean world. We trace the history of its observation, from Galileo Galilei's first telescopic view in 1610 to the Voyager flyby that revealed a puzzlingly smooth surface. The central focus is on how measuring an induced magnetic field finally proved that a vast sea of liquid saltwater exists beneath the ice, reshaping our understanding of the moon's geology and p ...