Where is the Ground on the DC PCB


Here is a picture from the Padhacking thread.


But I cannot seem to find the ground in that picture.

I know that one wire goes from the pushbutton to the corresponding solder point that is indicated on the picture. But the pushbutton has two prongs, I believe the other is the ground, where should I put that wire.

Can I run one wire thorough all the grounds and put it into the common ground when I find it?