It's not always shooters. Chemical leaks, refinery problems, and fires have all prompted authorities to ask citizens not leave their homes. Today's shutdown of Boston and surrounding towns is exceptional in scope — but the request itself isn't that uncommon.

A quick survey of Google News reports over the past two decades presented a number of instances in which municipalities or colleges called for the same sort of safety precaution that's taking place in Boston right now. The incidents we identified are plotted below.

The events can be broadly categorized. Shelter-in-place alerts driven by shootings or bomb danger (red markers), problems at oil refineries (blue), chemical leaks or spills (green), and fires (yellow). Each of the plotted points is described below: