<p>This article examines how coastlines—a spatial and symbolic threshold between land and water—can reshape approaches to urban edge studies and counter-mapping. Building on critical, speculative, and deep mapping, the article proposes the coastline as an interdisciplinary marking system that intersects urban geography, data studies, and design. Drawing on examples from co-mapping workshops of Coastliner Lab, the practice led by the author, and the Istanbul Coastline Atlas, a public digital archive of lost and transformed coastlines, tracing coastlines reveals how wetlands, quays, streams, and basins have been buried, privatized, or reconfigured by large-scale infrastructural developments in the top-down urban regimes of the city. This method is argued to raise further questions about ecological costs and hydro-social relationships. The article argues that coastline counter-mapping is both diagnostic and generative, making environmental change visible while providing shared, geospatial tools for more equitable coastal futures in Istanbul and beyond.</p>