Picture of a villa with an underground courtyard

Quick answer：

An underground courtyard in a villa is a garden or courtyard that is situated below the ground level of the building. These courtyards are often designed to create a sense of seclusion and privacy, providing a serene space away from the main living areas. They may feature various landscaping elements such as gardens, water features, and seating areas. The design of such courtyards can vary greatly, from modern minimalist styles to more traditional or elaborate designs.