Natural common open space. A minimum of 15% of the total site area shall be set aside and preserved for natural common open space. Natural common open space shall be designated, maintained and provided throughout and/or surrounding the PRD. An area designated, provided or maintained for natural common open space shall be in its natural, undisturbed condition or its maximum, restored undisturbed condition. Where significant or unique natural amenities exist on the site, the Borough Council shall have the authority to enforce and require the preservation of such amenities as part of the required natural common open space. These amenities may include, but are not limited to, natural features, such as rock outcroppings, virgin timber, woodlands, ravines, ponds, stream beds and stream valleys. Areas of land where underground facilities and/or utilities are located, and the areas used for stormwater retention ponds may be used for natural common open space. Hiking, biking and exercise trails may be permitted in the natural common open space, provided they are not impervious to water. This area may include the required buffer areas as well as nonbuildable areas of the site.