Iron-oxide concretions – curious-looking dark spheres – can be found by the hundreds of thousands in the dramatic red and white outcrops of Navajo Sandstone in Utah. The concretions typically consist of a sandstone core surrounded by a thick iron-rich rind, and they have an interesting geologic history and Native American cultural history. They also resemble Martian spherules or "blueberries", and may share similar origins.
Evidence indicating where ancient people collected the valuable pigment red ochre thousands of years ago may seem like an unlikely combination with underwater archaeology – but that isn’t the case. Sea levels have been substantially lower than at present for most of human existence, so we know that many traces of our history are hidden beneath water.