The Hadean is the geologic eon before the Archean. It extends back to the Earth's formation, and ended roughly 3.8 billion years ago (3800 Ma), though the date varies according to different sources. The name "Hadean" derives from Hades, Greek for "unseen" or "Hell" and suggesting the underworld or referring to the conditions on Earth at the time. The geologist Preston Cloud coined the term in 1972, originally to label the period before the earliest-known rocks. W. B. Harland later coined an almost synonymous term: the "Priscoan period". Other older texts simply refer to the eon as the Pre-Archaean, while during much of the 19th and 20th centuries, the term "Azoic" (meaning without or before life) was commonly used.