Drill cores of sedimentary rock which contains microscopic fossils. Maxwell Lechte Stored in an open-air warehouse in tropical Darwin, Australia, are dozens of trays containing cylindrical cores of rock. They are from drill holes bored hundreds of metres below the surface by mineral exploration companies decades ago. Some of these cores at the Northern Territory Geological Survey are mudstone – a type of sedimentary rock formed from hardened seafloor mud. The companies that drilled these cores