The Black Tortoise is painted on the northern wall of the Great Tomb of Gangseo. Unlike the other ethereal animals, the Black Tortoise's Chinese name Hyeon-Mu was derived from the black northern cardinal point color (Hyeon) and its rugged shell which resembles armor (Mu).
The Black Tortoise is depicted entwined with a snake, and together they symbolize the male and female genders. According to Chinese mythology, tortoises are exclusively female and must be paired with snakes, which have similar shaped heads, to conceive. Therefore, the adroitly entwined tortoise and snake do not represent a violent struggle, but rather a harmonious balance of Yin and Yang.
The Black Tortoise in the Great Tomb of Gangseo displays an entirely new line of symbolic features previously unseen in the Pyongyang region. The biggest difference is that the tortoise's head is turned to be face-to-face with the snake. Rather than having the snake wrap completely around the tortoise's abdomen, its tail exits between the tortoise's hind legs to create a smooth oval and form a knot with the head. This distinctive feature was previously unseen in any other depictions of the Black Tortoise. Additionally, the number of times the snake wraps around the tortoise had been reduced to just once.
This type of iconography is easily found in tomb murals of China's Southern and Northern Dynasties and is a valuable resource for understanding the cultural exchanges between Koguryo and various Chinese states of the period. However, the tension in the curve formed by the snake isn't as apparent in Chinese depictions of the Black Tortoise, and thus the harmony created by the tortoise and the snake in the mural of the Great Tomb of Gangseo is rather unique. The Black Tortoise in the Great Tomb of Gangseo expresses the tortoise's stability and the flexible curves of the snake in exquisite harmony, and this rendition can be considered not only the finest example of the Black Tortoise from Koguryo, but throughout East Asia as well.