Activate the eclipse.
Go to the tunnel, which contains a statue needed to open PaP, near the Pharo. A valve is located in it, you will have to interact with it 4 times to turn it. You must now lure a napalm zombie into three gas leaks in the basements.
They are located at the following locations:
1) Next to the valve you just turned
2) Next to the geyser
3) Before the room with the power generators
Once done, go back to the valve. There is a lever besides it, push it down to complete the step.