Making a hole in a skull ????

Read this:

Notice circled paragraph in red then you'll know what's the term "Trepenation".

"Trepanation is the surgical term for making a hole in the skull bone. Doctors more commonly use the term craniotomy (skull opening). It is the first stage of every brain operation..."

There's a group that advocates making a hole in a skull - International Trepanation Advocacy Group (ITAG).

Why is the reason of this hole drilling on the head?

One main reason is to increase brain blood volume which is believed that it improves one's brain function and ablity if the individual has an hole on his/her head.

Read more about it in ITAG's Trepanation FAQ