Mat Than Mountain

A round hole opens clean through a limestone mountain in Cao Bằng, from one face of the rock to the other. There are many kinds of eyes in the world, but few look straight through a mountain.

The Vietnamese name for this mountain is Mắt Thần Núi, the Divine Eye of the Mountain. The hole is more than 50 meters across. It sits about 50 meters above the surface of Thang Hen Lake. The Tày call the mountain Phja Piót, which means the pierced mountain. The Tày are one of the peoples of northern Vietnam.

The Department of Cultural Heritage describes it as a round opening that passes through both faces of the mountain. It calls the place a distinctive scenic spot in Vietnam. On June 29, 2021, the Ministry of Culture, Sports and Tourism signed a decision ranking it a national scenic site. The province's 2024 inventory of heritage sites records it as unchanged from its original state. It lies in Thăng Sặp hamlet, part of Cao Chương commune.

The Eye is one place within the Non Nước Cao Bằng UNESCO Global Geopark. A UNESCO Global Geopark is an area with landscapes of international geological importance, managed for protection, education and sustainable development. UNESCO says the limestone ranges here show how limestone country evolves in the tropics, a process called karst. It singles out the Thang Hen lake system, where 36 natural lakes connect with one another, as showing how water moves through karst. The Eye lies within the area of that lake system.

The hole itself is the work of karst. The limestone is full of cracks, and water has worn at it for millions of years. The water found its way through the gaps and widened them little by little. At last a passage ran right through the mountain. When the hole grew large enough to let light pass, the mountain became what the Tày call Phja Piót.

When international scientists prepared the file for the geopark, they gave the mountain an English name, Mountain Angel Eye. A 2021 article in the VnExpress newspaper records this international name. But the name under which the Vietnamese state ranked the mountain is Mắt Thần Núi.

The ground below the mountain changes with the seasons. According to VietnamNet, the valley at the foot of the mountain is a wide meadow in the dry season. In the rainy season the water rises, and the whole valley can turn into a lake. The Cao Bằng Department of Culture, Sports and Tourism says the local authorities worked with the geopark's management board to build a viewing point at this scenic spot.

Standing in front of the Eye, you are looking at the result of a geological process that lasted millions of years. No person made this hole. There was only water, stone and time.

Through the circle, the sky appears framed by the mountain. In the dry season, a carpet of green grass stretches below, across a quiet valley. In the rainy season, the water rises over the whole valley, and the Eye looks down on a lake. It looks down as it did long before any person set foot on this land.