Talk:Grounders/@comment-28398720-20160901092902/@comment-26415383-20160901113002

Well in Fog of War, Octavia and Bellamy found reaper Lincoln but in Long Into An Abyss, Clarke says something along the lines of "I knew Mount Wheather controlled the reapers but I didn't know they were creating them."

So, likely in that episode was when they found out.