This sea trench is squeezed between two vertical rock faces, with a narrow line of sky above and the sea below, the sound of waves pounding against the rocks reverberating like a herd of stampeding horses. A tunnel has been built to connect the two cliffs, supported on a concrete bridge dozens of meters above the sea. But A Thread of Sky is located deep within a military control area, so before visiting you have to get a permit from the Dongyin Visitor Center and be accompanied by security soldiers during your visit.