"What is a stromatolite? he asks rhetorically, his eyes gleaming. The word comes from the Greek stroma, a mattress, coupled with the root word for "stone." Stone mattress: a fossilized cushion, formed by layer upon layer of blue-green algae building up into a mound or dome. It was this very same blue-green algae that created the oxygen they are now breathing. Isn't that astonishing?"