How Deep Is the Sea? Ocean Depth, Trenches and the Unmapped Sea Floor, from Open Data

I describe the depth of the world ocean from the open AWS Terrain Tiles, whose sea floor comes from ETOPO1, GEBCO and SRTM15+, using the zoom 4 level of the tiles as a 4,096 by 4,096 Web Mercator grid of about 9.8 km at the equator, with every cell weighted by its true area and by the share of it that is sea. The sea covers 362.9 million km², 71.2% of the Earth, with a mean depth of 3,675 m, a median of 4,089 m and a volume of 1.33 billion km³, close to published estimates. Continental shelf shallower than 200 m covers 7.7% of the sea floor, slopes, rises and ridges between 200 and 3,000 m 17.3%, the abyssal depths between 3,000 and 6,000 m 74.1% (more than half of the planet's surface) and the hadal trenches below 6,000 m 1.0%. The Pacific is the deepest ocean on average (3,943 m) and the Arctic the shallowest (1,205 m). Comparing the grid with published surveys of 14 of the deepest places shows how much a 10 km grid smooths the trenches: its deepest Mariana cell is 671 m shallower than the 10,935 m measured at the Challenger Deep, its deepest cell overall lies in the Tonga Trench, and in 4 trenches it is deeper than any measured depth, pointing to errors in the source grids. National waters cover 39% of the sea but hold 37% of the floor deeper than 6,000 m. Finally I set these numbers against the Seabed 2030 project's estimate that only 28.7% of the floor is mapped to modern standards. An interactive map draws the same tiles at every zoom with a depth readout for any point. Interactive version: https://maps.tristanhotham.com/ocean-depth/. Data and code: https://github.com/Tristan-Hotham/ocean-depth. Blog post: https://blog.tristanhotham.com/2026/10/31/ocean-depth/. Text, figures and derived data: CC BY 4.0; code: MIT. Depths: AWS Terrain Tiles (Mapzen; under the sea ETOPO1, GEBCO and SRTM15+, sources listed at github.com/tilezen/joerd), not redistributed. Sea areas, Exclusive Economic Zones and undersea feature names: Marine Regions (VLIZ), CC BY 4.0. Land, lakes and bathymetry polygons: Natural Earth (public domain).