This visualization displays world population density through colored contour layers on a Natural Earth projection map. City population data from the Simplemaps World Cities Database is weighted to compute density contours using `d3.contourDensity`. Colors interpolate between `interpolateCubehelixDefault` and `interpolateReds` based on density values, rendered via `d3.geoPath` with land clipping. The map supports zooming through `d3.zoom` and includes city labels for populations above seven million.