The Culling, also known as Kane's Population Reduction Plan, was an event spearheaded by the Council where 320 Arkers willingly sacrificed themselves so the rest of the Ark would have more time to repair their life support system.
After the Delinquents were able to contact the Ark and let everyone know the Earth was able to support life, the families of those culled became upset with what they now perceived as an unnecessary sacrifice. This contributed to the Unity Daymutiny.
In a flashback in Earth Kills, a year prior to The Culling, Jake Griffin discovered a system failure on The Ark, leaving them with less than two years of oxygen which was not enough time to fix the problem. The Council ordered Jake to remain quiet about it to keep the public from panicking but he created a video anyway. The Council stopped him before he could broadcast his warning to the Ark and had him floated for treason.
A year later, the oxygen shortage on the Ark had become critical and the Council agreed to send 100 Delinquents to Earth in the Pilot to not only see if it was survivable but also to give the Ark another month's worth of oxygen by reducing the population. It was still not enough as the Ark would run out of oxygen within four months while Engineering still needed another six months to fix life support. A day later, in Earth Skills, the Council voted on Kane's Population Reduction Plan to reduce the population by 209 individuals. Aware that for every day they delay, ten more people will need to be culled, the vote ended in a tie with Chancellor Jaha abstaining from the vote, causing it to be taken up again in ten days.
Ten days later, in Twilight's Last Gleaming, as oxygen, CO2, and methane reached perilous levels and Medical was overwhelmed with cases of pulmonary toxicity, the Council voted unanimously on the Culling and agreed to euthanize 320 individuals to give the rest of the people on the Ark more time to get the problem fixed.
The original plan was to wait for Section 17 in Factory Station to be in their sleep period aided by a sleep-inducing gas. All of the doors and vents would be sealed in an alleged malfunction of their fire containment system. Chancellor Jaha planned to be culled with the people in Section 17, however, Marcus Kane convinced him it would be too destabilizing for the Ark to also lose their Chancellor.
Abigail Griffin did not agree with the plan to keep the Culling secret from the public and resurrected Jake's video, broadcasting it across the Ark that the life support was failing. In response, the citizens of the Ark banded together with Tor Lemkin being the first one of hundreds to volunteer for the Culling so their loved ones could live. In the end, the Council had to turn away almost one hundred extra volunteers.
The Culling happened in a small room with a fan. The door closed and the fan slowly stopped and the 320 people fell asleep before taking their last breaths.
Abigail Griffin was arrested and brought before The Council in Contents Under Pressure. They decided to not float her because her medical expertise was too valuable, however, they did decide to remove her from the Council. While she was before the Council, the first contact from the Delinquents was broadcast to the Ark over the Ark-wide channel. Not only did this let everyone know the ground was survivable, it also finally allowed the rest of the Ark to learn what the Council had been keeping secret from them and eventually contributed to the Unity Day bombing and mutiny.
A memorial to the victims was set up in Section 17 with the Last Tree and the book of The Ark Grounders' names. An inebriated and guilt-ridden Marcus Kane visited the memorial and was confronted by Cuyler Ridley who had lost his wife in the Culling. Chancellor Jaha then ordered a General Assembly of the Station Representatives to finally tell everyone the truth about the Delinquents on Earth and the failing life support system on the Ark. During this meeting, he elected former Chancellor Diana Sydney to take Abby's place on the council. This eventually led to Sydney's failed coup in Unity Day that left the Ark further crippled and resulted in the deaths of around 1,500 Ark citizens, according to Kane in The Calm.
Marcus Kane: "We don't have time. Engineering needs 6 months to fix life support, and we'll be out of oxygen in 4. Nobody wants to do this, but the inescapable fact is that for every day we delay, 10 more people will need to be sacrificed. So today it's 209, tomorrow it's 219. The day after that is 229. We're the ones who need more time. I move that we vote now."
Council member: "Very well. The matter before us requires a 4-vote majority to pass. A vote in favor is to vote to cull 209 citizens of the Ark from the supply grid in order to extend life support for those who remain by 6 months."
Marcus Kane: "It's essential to maintaining public order after the culling takes place. All residents of Section 17 will be in their sleep period. The Council has already issued a writ of consent giving the Chancellor permission to carry out the population reduction. A sleep-inducing gas will be introduced into the air. On your command, the oxygen supply to Section 17 will be cut off. It should be painless. Document needs your approval."
Jake Griffin (on video recording): "Time is running out. This is an undeniable reality, but we have pledged our lives to make sure that humanity does not share that fate. Now, while there is still time, we must come together and face this crisis head on. I believe it will bring out the best in us our strength, our humanity, our faith that we will come together in this time of uncertainty."
Abigail Griffin: "My husband was killed for trying to warn us. In 12 hours, 320 people will be sacrificed to extend our oxygen supply unless we take action. The future doesn't belong to the Chancellor or The Council. It belongs to all of us."
Diana Sydney: "The workers. They're coming to me with dangerous questions, Thelonious. Many of them lost loved ones in the culling. The belief that your administration let people die when there was another way has taken root."
Chancellor Jaha: "You are the Station Reps of The Ark, and I don't blame you for losing faith in me. I kept The Ark's oxygen crisis from you. I lied about sending the hundred to the ground. But you can tell your stations that the deception ends here. You now know the whole truth, but there is one truth that stands above all the rest. Earth is survivable. The 320 souls who sacrificed their lives in the Culling gifted the rest of us the time that we need to make it to the ground. Mankind is going home."
Diana Sydney: "My friends, please. Chancellor Jaha has sacrificed as much as any of us. But his loss, just like all of yours, is not in vain. The Chancellor's son died finding out that Earth is survivable. And because of your wife's sacrifice, your daughter will get to come down to the ground. That is not in vain."
Diana Sydney: "This launch should be delayed at least until you've questioned the terrorist that set that bomb. I put word out to my people and just heard back. His name is Cuyler Ridley. He's a mechanic who lost his wife in the culling."
Cuyler Ridley (to Kane): "I lost my wife. You're damn right I'm upset, but this is about more than the Culling. Workers all over the Ark are tired of getting screwed. It's our kids you sent to die on earth, and who's on the first drop ship? Not us. Over and over, Jaha and rest of you on The Council betray our trust. Today is your reckoning."