If you are referring to the way the land sags in the middle of the continent (to below sea level, in fact), this is largely because there is an ice sheet many thousands of feet thick sitting on top of it.
Sunken is not a verb so it doesn't have present or past tense. Sunken is an adjective
generally xerophytic plants have sunken stomata
The Sunken World was created in 1948.
The Sunken Billions was created in 2008.
The Sunken Cemetery was created in 2002.
Sunken Treasures was created in 1993.
The opposite of a sunken area would be a raised area or an elevated area.
The Sunken Threshold was created in 2006-02.
Dehidration is a major cause in sunken veins.
his eyes are sunken in because he is a cat simple.......
"The sunken ship sat on the bottom of the ocean."
The Sunken Window goes in your house.