The Stalacpipe Organ

from wikipedia:

The Great Stalacpipe Organ was designed in 1956 by Leland W. Sprinkle inside the Luray Caverns near Shenandoah National Park in Virginia, USA.

Sprinkle created the Organ over three years by finding and shaving appropriate stalactites to produce specific notes. He then wired a mallet for each stalacite that is activated by pressing the correct key on the instrument's keyboard. The stalactites are distributed through approximately 3.5 acres of the caverns but can be heard anywhere within its 64-acre confines.