The Key to Humanity is a Side Mission (Side Quest) in Dying Light 2. This walkthrough will guide you through all objectives of The Key to Humanity side mission.
Region: Quarry End
Quest Giver: Dustin
Requirement: Activate the ‘Oak’ Windmill
Reward: 750 Combat XP, 150 Old World Money
Mission Info: A boy, the son of a scientist, asked me to find an incredibly important CD that he said could save humanity.
Objectives:
- Search the dark hollow for the mysterious CD
- Bring the CD to the scientist
Starting Location: The Key to Humanity
The Key to Humanity is a side quest in Quarry End that can be started by talking to Dustin at the Smokehouse after activating the ‘Oak’ Windmill. This is a night quest, so after starting it hold 
to fast-forward to night.
Search the dark hollow for the mysterious CD
Head to the dark hollow at the marked location in Horseshoe. It is advised that you go here at night as there will be far fewer infected inside the dark hollow at night compared to day.
You can get inside either from the street level or from the rooftop. Once inside, clear the area of infected and break through the wooden door to reach the kitchen. Picklock the hard door and you will find the CD inside a bag in this room.
Bring the CD to the scientist
Once you get the CD from the dark hollow, head to the Bazaar in Trinity to speak to the scientist.
This finishes The Key to Humanity Side Quest in Dying Light 2 Stay Human.
For all other Side Quests and Main Missions, check out the complete Dying Light 2 Walkthrough.
The bag is not there but instead when I use my survivor sense it’s saying it’s one floor up but there is no way of getting there. Could this be a bug
This quest along with a bunch of others decided to bug/dissappear after I left the first part of the city. Do the side quests in the first part of the city BEFORE leaving for the second part of the city ASAP.