Strava is an application for athletes through which you can track the training conducted among other functions. The developers of this application have published a tool called heat maps that shows the combined activity of all users in different parts of the world.
The problem is that the heat map shows the route taken by users in especially sensitive sites, such as secret military bases. Thanks to these heat maps you can see, for example, the routes made by the personnel of a military base or create a plane of it.
From Strava, they indicate that users who wish to exclude certain places from heat maps should mark the activity as private.