The release of the Google Maps API in 2005 spurred a trend of mapping mashups, adding cartography to online participatory culture. This article will present how the affordances of these “platforms” give shape to the online participation of concerned citizens willing to access information during an environmental crisis. Based on the analysis of the radiation mashups created after the disaster at the Fukushima Daiichi power plant in 2011, this article will highlight two types of online participation. First, participation as data extraction, where concerned actors either monitored data using Geiger counters or extracted and republished data from official websites. Second, participation as data aggregation, where maps were used to display and compare radiation measurements from official or crowdsourced venues. The conclusion will highlight the necessity to study how online platforms assign a place and temporality to online participation.