Invited by Milan Design Week 2018 and Interni, MAD Architecture, led by Ma Yansong, exhibited an art installation called "Sky Mirror" in the Seminario Arcivescovile (Catholic Seminary of the Archdiocese of Milan), a historical building from the 16th century in the old city of Milan. This circular art installation redefines the dimension of space from different angles. In traditional three-dimensional space, four walls define the boundaries of space; while this installation, which seems to be nothing and "floating" in the air, seems to be the "fifth boundary" between space and heaven and earth beyond the four boundaries.

The waterscape ground in the courtyard is as bright as a mirror. The circular device jumps into the water, and the reality and reflection alternate between reality and illusion, making it difficult to distinguish the boundaries. When people walk in the space, the picture is full of emotions and scenery, as if they are walking on the water or walking in the clouds. Through the device, people look at the sky from the ground, as if looking at the vast and endless universe, and pay more attention to their own hearts, as well as the interaction and dialogue between heaven and earth.