Deep underground

 is a Japanese concept for the public use of deep underground, enabled by a 2001 law. It was first thought of in the late 1980s as Japan faced ever-increasing land values in the economic bubble, to allow the use of heretofore-unused deep underground for necessary water and utility ducts and other city tunnels.
However, due to technical difficulties in ventilation, emergency procedures and other safety-related issues and the relatively high cost, no projects using deep underground have been completed as of 2010.
Definition.
In the case of public use, no compensation to the land owner is required.