Source: Narrative Visualization: Telling Stories with Data
This structure gives a considerable amount of control to the reader: as an author, you don’t control the order in which the user will be exposed to the content blocks of the story. The role of the author in this structure is to curate the substories to include, what details these stories will contain and how the stories reference each other and the way they can be navigated.
An example of a drill down map is the global map of Freedom House. Clicking on a country reveals the key scores for each country in a pop up, together with a link to the country report of the selected country.
Source: freedomhouse.org/explore-the-map