Underground metro systems: a durable geological proxy of rapid urban population growth and energy consumption during the Anthropocene

For the ~541 million years of the Phanerozoic Eon, trace fossils have provided a distinctive pattern of marine and terrestrial ecosystem evolution. From about 10,000 years ago, this pattern was supplemented with increasingly widespread human-made trace fossils, which eventually developed to a scale and complexity unlike anything from previous geological history. Most recently, during the past two centuries, some types of human trace fossil have become nearly global in their distribution. These trace fossils are exemplified by underground metro systems beneath cities, first developed in the 1860s, and reaching a near-global spread in the second part of the twenty-first century, approximately coincident with the Great Acceleration. Metro systems have grown in tandem with urban populations and energy consumption. They are both a geological proxy of the change of Homo sapiens from a rural- to dominantly urban-dwelling species, and a signal of highlyconcentrated energy consumption by humans in urban areas, estimated as 371 EJ (1 EJ=1018 J) in 2010. This paper focuses on geological aspects of metros whilst recognising that they are also structures of archaeological significance. Penetrating deeply down from the urban strata of the nineteenth to twenty-first centuries, metros chronicle specific aspects of the growing human impact on the Earth system, and as such provide a durable marker to geological deposits associated with a potential Anthropocene Epoch of geological time.