The.100.S03E09.720p.English-

For the 100, Earth is an alien planet; however, they quickly discover that Earth is filled with new wonders and dangers of all sorts. Then, they discover that not all humanity was wiped out, and some survived the nuclear apocalypse: the Grounders, who live in 12 clans locked in a power struggle; the Reapers, another group of Grounders who have become cannibals; and the Mountain Men, who live in Mount Weather and locked themselves away before the apocalypse.
In the third season, Camp Jaha, now renamed Arkadia, comes under new management when Pike, a former teacher and mentor, is elected over Kane as Chancellor and begins a war with the Grounders. An artificial intelligence named A.L.I.E. was revealed to be responsible for the nuclear apocalypse that devastated Earth 97 years before the series, and she takes over the minds of nearly everyone in Arkadia and Polis – the capital city of the Grounders. In the third season finale, Clarke manages to destroy A.L.I.E. after it is revealed that the world is facing another nuclear disaster. This is due to nuclear reactors around the world that have fallen into disrepair and are irradiating Earth, once again threatening to make it uninhabitable.