What you need to know about the inside story of how the company allowed PFOS to seep into all of us while sitting on research that showed the chemical is toxic.

After years of reporting on forever chemicals, ProPublica reporter Sharon Lerner had one question that still nagged at her. She knew that a handful of 3M scientists and lawyers had learned in the 1970s that the chemical PFOS had seeped into the blood of people around the country and that company experiments around that time had shown that PFOS was toxic.

But the company kept making the compound until 2000. How, she wondered, had 3M kept its dark secret for decades? For years, no one who knew what had happened inside the company had spoken publicly.

Then last year, a former 3M chemist reached out to Lerner.

Here are nine takeaways from the investigation published by ProPublica and The New Yorker.