This example displays a world map using a Mercator projection, with each country rendered as a gray shape outlined in white. The map is built with the D3.js version 3 library, using `d3.geo.mercator()` to define the projection and `d3.geo.path()` to convert GeoJSON geometries into SVG paths. The geographical data comes from a `countries.json` file containing feature collections loaded via `d3.json()`. The resulting visualization, rendered in an SVG element, demonstrates the classic distortion of landmasses toward the poles inherent to this projection method.