Gravissima
Gravissima is an organ stop that is usually pitched at 64'. It is usually a resultant (a 32' stop combined with a 212⁄3' stop, which is a fifth, producing a difference tone of 8 Hz on low C.) It is also usually the lowest stop in the organ that has one.
Tonal quality
This is usually a quiet stop, producing a deep rumble, often used to create a profound effect in music.