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. With Hardenberg as our guide, we traverse the muddy spaces of Venice and Amsterdam, the coasts of the Baltic Sea, the Panama and Suez canals, and the Himalayan foothills. Born out of Enlightenment studies of physics and quantification, sea level became key to state-sponsored public works, colonial expansion, Cold War development of satellite technologies, and recognizing the climate crisis. Mean sea level, Hardenberg reveals, is not a natural occurrence—it has always been contingent, the product of people, places, politics, and evolving technologies. As global warming transforms the globe, Hardenberg reminds us that a holistic understanding of the ocean and its changes requires a multiplicity of reference points. A fascinating story that revises our assumptions about land and ocean alike, Sea Level calls for a more nuanced understanding of this baseline, one that allows for new methods and interpretations as we navigate an era of unstable seas.