The third important physiographic feature is the karst region in the north. This area consists of formations of rugged volcanic rock dissolved by water throughout the geological ages. This limestone region is an extremely attractive zone of extensive mogotes or haystack hills, sinkholes, caves, limestone cliffs, and other karst features. The karst belt extends from Aguadilla, in the west, to a minor haystack hills formation in Loíza, just east of San Juan.
And... if you'd like to learn ALL about karst, and see some good photos: look here