Head Gasket replacement. What may have caused this damage?

Hi,

I was doing a head-gasket replacement in my 4runner 3rd Gen 5VZ and found what I consider a huge issue in the head, to be more specific at cylinder #6, in theory this is the first time opening this engine, but I found a serious damage in cylinder 6. Any ideas of what may have caused this damage. There was pressure and bubbling in the coolant:
1. The dent can be feel by hand, it is notorious, not sure if that can be fixed by removing some of the head material, it is like 1mm depth... Mostlikely this head is useless now. Do you agree?
2. Is this kind of damage caused by bad coolant or not changing the coolant over a extender period? I have the car for 5-6 years now with periodic coolant changes, but there were previous owners and it is not guarantee that they took good care of the coolant

Thank you in advance for your comments!

 

I'm guessing you could mill (plane) about half of it out? Is it really 1mm?

Take it to a machine shop or buy a reman head?

It doesn't seem coolant related to me, but could be.