Appears that a 4.4 billion-year-old crystal is oldest piece of Earth, and the crystal's chemistry -- specifically, the ratio of oxygen isotopes within it -- suggests that the temperatures on Earth 4.4 billion years ago would have supported liquid water, and therefore perhaps life; Earth is 4.54 billion years old.
According to this xkcd comic, "all the crust older than 3.5 billion years has been recycled into the mantle by subduction."
Is it true that it is possible there is a roughly one billion years of Earth's history with water present that's unaccounted for, and if so, do we have any reason to believe we'll better understand this time in Earth's history?