An "adverse event" is when something goes wrong with a patient in a hospital. It can be as commonplace as bedsores, or as dangerous as "retention of a foreign object" after surgery.
Hospitals are required to report these events soon after they happen. Many states, like Washington or Minnesota, publish these numbers regularly. California publishes these statistics in a single annual report. The Investigative Unit filed a Public Records Act request to find out which hospitals had what kinds of medical errors.
In the map above, each circle represents a hospital. Zoom the map to a hospital near you and click the circles to see which "adverse events" have occurred there in the past four years.