Zircons from the Jack Hills in the Narryer Gneiss Terrane, Yilgarn Craton, Western Australia, have yielded U–Pb (uranium–lead radioactive dating) ages up to 4.404 billion years. This is judged to be the age of crystallization, making them the oldest minerals so far dated on Earth. In addition, the oxygen isotopic compositions of some of these zircons indicate that more than 4.4 billion years ago there was already water on the surface of the Earth. This interpretation is supported by additional trace element data, but is also the subject of debate.