The limestone of the hills is actually called karst. This topography is caused when layers of bedrock made up of a soluble substance such as dolomite or in this case limestone. The landscape has been slowly eroded through a process called solvation. The hills are that which is left behind.
From the Upcoming ueue, submitted by taliesyn30.
My mom got to go to the Philippines last year to visit her relatives, and she took pictures of her leaping on top of one of the hills (during the season when it's all lush and green, of course).