Sinkhole may exhibit a visible opening into a cave below. In the case of exceptionally large sinkholes, such as the Minyé sinkhole in Papua New Guinea or Cedar Sink at Mammoth Cave National Park in Kentucky, an underground stream or river may be visible across its bottom flowing from one side to the other. Sinkholes are common where the rock below the land surface is limestone or other carbonate rock, salt beds, or in other soluble rocks, such as gypsum, that can be dissolved naturally by circulating ground water. Sinkholes also occur in sandstone and quartzite terrains. As the rock dissolves,
The Heart Is a Lonely Hunter (Once Upon a Time) that Graham helped fill a void for her character, commenting that his loss "was not easy for [Regina]. That’s why, when she crushes his heart, there’s a tear coming out of her eye. She didn’t want to do it, but she had no choice — almost like how she had no choice to kill her father. I’m not saying I agree with it, but for the character, it’s what she had to do." Parrilla also cited "The Heart Is a Lonely Hunter" as the first episode where viewers see "how the two worlds collided," a reference to Regina's ability to