Without having ever heard one in real life, I know that the octobass is already one of my favorite musical instruments. It is the largest member of the stringed instrument family that includes violas, violins, cellos, basses, and double basses. The octobass is 11 ft tall, has three strings, and must be played by pedals and levers due to its large size. The lowest note it can play is around 16 Hz. This is below the range of human hearing so the note is only felt, not heard. You can, however, hear the overtones of this note. There is only one orchestra that currently uses the instrument: The Montreal Symphony Orchestra. Click here to learn more.
To see a video by a curator click here: Link
Note: It will sound funny through small computer or phone speakers because you would need to use a music system with very powerful subwoofers to truly hear (and feel) the octobass. However, even with small speakers this recording of a tuba and an octobass playing together sounds pretty good: Link