With the Japanese government only providing spotty information about the radiation leaking from the damaged Fukushima nuclear plant in the early days after the devastating March 11 earthquake and tsunami, a group of tech-minded citizen scientists set out to fill in the “black holes” in the knowledge base.

They did so by crafting their own Geiger counters and handing them out to volunteers in the disaster area to measure the fallout. Months later, they have assembled thousands of radiation readings plotted on maps that they hope will one day be an invaluable resource for researchers studying the impact of the meltdown at the crippled nuclear complex.

Safecast took its first reading on April 16. Today, it has about 50 regular volunteers who collect data from their homes or while driving, build devices or assist in other ways. Those using vehicles equipped with Geiger counters cover an area that Franken estimates to be about 620 miles long by 185 miles wide. To date, they’ve collected 251,000 data points from their drives and fixed reporting stations, and have received about 60,000 more from other sources, including people with their own Geiger counters.

Safecast publishes the data on its website and publishes it to a number of other places so the information can be used by the greatest number of people, Bonner said. It also aggregates radiation data from a number of sources, including the Japanese government.