Earth's first 150 million years were defined by intense bombardment and seas of molten rock. As this Hadean chaos subsided and the surface cooled, an atmosphere dense with water vapour gave way to millions of years of rain. The oceans accumulated drop by drop.

The origin of ocean water is debated. One hypothesis holds that water was released from silicate minerals within the planet itself through volcanic outgassing. Another points to the carbonaceous chondrite meteorites and icy comets that pelted the early Earth; isotopic analyses suggest both sources contributed.

Almost no direct record of this era survives as rock — but zircon crystals from the Jack Hills region of Western Australia offer a window into that vanished world. Dated by radiometric U-Pb methods to 4.404 billion years, these microscopic crystals carry oxygen isotope ratios and low-temperature chemical signatures that indicate the magma from which they crystallised had interacted with liquid water. In other words, the oceans left their trace in these tiny crystals long after every surrounding rock had eroded and recycled.

The earliest oceans were far more acidic than today; dissolved CO₂, H₂S, and other volcanic gases saturated the water. Over time, ocean-atmosphere gas exchange and rock-water interaction gradually shifted the balance, raising pH toward more neutral values. This neutralisation was not only a chemical milestone but a biological one: early microbial life required — and in turn further altered — a chemically stable aqueous environment.

Without oceans, life as we know it would almost certainly not have arisen. Water concentrates molecules, creates energy gradients, and enables the organic chemistry that underlies biology. The hydrothermal vents of those first ocean floors — dark, hot, and mineral-rich — may have been the stage on which that story began.