Traces a commonplace average-sea level-from its origins in charting land to its emergence as a symbol of global warming. News reports warn of rising sea levels spurred by climate change. Waters inch ever higher, disrupting delicate ecosystems and threatening island and coastal communities. The baseline for these measurements-sea level-may seem unremarkable, a long-familiar zero point for altitude. But as Wilko Graf von Hardenberg reveals, the history of defining and measuring sea level is intertwined with national ambitions, commercial concerns, and shifting relationships between people and the ocean. ‘Sea Level’ provides a detailed and innovative account of how mean sea level was first defined, how it became the prime reference point for surveying and cartography, and how it emerged as a powerful mark of humanity’s impact on the earth.