Lake Matsu is roughly circular, with one very large island located on the west side of the lake halfway between the shore and the center of the lake.
Large natural gravity Dust deposits along the south shore of Lake Matsu form huge crystals that cause small islands to float several hundred meters above the lake. The Gravity Dust is known to cause turbulence for ships flying through the area. While many of these islands are nothing more than large rocks, some of the islands are large enough to support grass and even trees. This area is home to dangerous Grimm, including Lancers.
Lake Matsu's exact size is unknown, but in "Welcome to Haven", the cargo airship that Weiss Schnee is traveling on receives a distress signal from an airship under attack by the Grimm. In the message is the phrase "The Grimm hit us 400 klicks north of the southern shoreline". If the nearest shoreline is 400 km away, then Lake Matsu must be at least 900 kilometers in diameter, if not larger.
- Matsu can refer to several different things in Chinese. It is the name of a sea Goddess, the Japanese Pine tree and an archipelago of islands.