Octobass

Composers and orchestras like to use their lowest instruments sparingly, for maximum impact. Few instruments go lower and are used more sparingly than the huge and strange octobass, one of the rarest classic instruments in existence.

Invented in 1850 by Jean-Baptiste Vuillaume, the octobass was intended to bring an extremely deep rumble to the orchestra sound. The three-stringed instrument stands between 11 and 12 feet tall, about twice the height of a double bass. This giant bass produces sound so low, some of the notes fall outside the range of human hearing—these vibrations can only be felt.

The rare instrument is almost too large to play. The strings are too big to press with your fingers, so fretting them requires operating pedals and levers that control capo-like mechanisms to press down the strings. In fact, the octobass originally required two people to play: one on the bow (which, though shorter that the bow of a double bass, is extra heavy) and one working the lever system. 

Vuillaume originally built three octobasses, two of which are still around. Including contemporary, playable replicas, there are only seven known examples of the instrument in the world, mostly kept in museums, including one at the Musical Instrument Museum in Phoenix, Arizona. The octobass in Phoenix is tuned C0, G0, D1, a range that's two octaves below the cello and one octave below the modern double bass. Its low C note is lower than the lowest note on an 88-key piano, and beyond what most people can hear (you can give it a listen here).

The Montreal Symphony Orchestra is the only orchestra that owns one of these unusual instruments, and composers still write music for it on occasion. When played today, it is usually by a single person.