No scientist has ever directly observed an eel laying eggs in the wild. Each autumn, adult eels undergo a dramatic transformation: their yellow bellies turn white and silvery, their eyes grow larger, and their skin thickens. This stage is known as the "silver eel" phase, and it signals that the eel is preparing to migrate toward the Sargasso Sea, a vast area in the Atlantic Ocean. Despite more than a century of research, no adult eel has been seen laying eggs in nature. The European eel, in particular, is critically endangered and could vanish before its mysterious reproductive behavior is fully understood. The migration of silver eels typically happens in autumn and at night, often during periods of flooding, which helps them avoid predators. Once they begin their journey, they stop eating and rely on fat stores built up during their time in freshwater. The Sargasso Sea is believed to be the eel’s breeding ground, based on the discovery of larvae in the area since 1904. However, no adult eels have ever been observed in this part of the ocean, and no reproductive acts or fertilized eggs have been recorded. Scientists have used satellite transmitters to track the long-distance migration of silver eels, following their journey across thousands of kilometers. These studies have provided valuable insights into their movement patterns but have not yet confirmed the exact location or process of reproduction. For the American eel, a close relative of the European eel, no sexually mature adults have been found in the Sargasso Sea, and its reproductive behavior remains a mystery. Researchers have inferred the timing and location of spawning by studying the tiny ear bones of eels, called otoliths, and using oceanographic models, but no direct observations of the act itself have been made. The European eel is currently listed as "critically endangered" on the Red List of freshwater fish in Metropolitan France, as established by the International Union for Conservation of Nature (IUCN) in 2019. Understanding how eels reproduce is vital for their conservation, as the species could disappear before its reproductive secrets are uncovered. Without this knowledge, efforts to protect and restore eel populations may remain incomplete.