To learn more about how dark money groups spent the money they were secretly raising, ProPublica launched the “Free the Files” project. First, we enlisted volunteers to gather files from local TV stations detailing political ad buys and share them with us to release online. These records, previously available only on paper to people who visited the stations in person, provided details about spending often not included in the groups' reports to election authorities. The FCC later ordered some stations to put ad buy data online, but the jumble of documents posted could not be digitally searched. To make the information useful, ProPublica created a news application that showed volunteers how to sort the records by market, amount and -- most critically -- by candidate or group. More than 1,000 people helped us create a database that logged up to $1 billion in ads. Tapping this data, ProPublica reporters were able to show how massive infusions of dark money influenced races in Ohio and New Mexico.