The concept of "art in mid-air" is quite subjective and can be interpreted in different ways. However, if one were to consider it as a phrase describing a particular style of art or technique, its antonyms could be grounded art or static art. Grounded art refers to artwork that is created on solid surfaces, such as paper or canvas, and often requires physical contact with the ground or a stable support. On the other hand, static art conveys a sense of stillness and may not involve any physical movement or suspension in the air. Both these antonyms contrast the idea of art hanging or floating freely in space, which characterizes the concept of "art in mid-air.