I am very skeptical on the fact that the rock didn't hit anything due to the so-called easterly deviation initially pointed out by Isaac newton ... I am thinking it's not that deep and there was mud at the bottom and it suppressed the sound ...
This is a interesting phenomenon if you are not aware of it ... ''If an object let's say a throwing dart to avoid arguments around air dynamic and frictions is drop in a very deep shaft it will always hit the east wall due to diurnal rotation of the earth on its axis. Because of this earth rotation a body dropped from a fixed position will always deviate eastward of the vertical along which it would otherwise fallen. It might seem at first thought as if the body should depart from the vertical to the westward rather than to the eastward since the earth rotates in the latter direction. However, the so-called fixed position is fixed only with reference to the earth’s surface, and the object before release partakes of the same motion, and has the same velocity, as all other “fixed” objects in its neighborhood. Were it at the equator this speed would be say thousand miles per hour. The speed grown less toward the poles, where it becomes nothing. In fact the object is traveling in the circumference of the circle of latitude in which it happens to lie. Points beneath it have a speed which is smaller as the (sic) lie nearer the axis of rotation. The freely falling body retains the eastward speed with which it started, and so gains on the slower moving parts of the earth which it is approaching. It, therefore, moves eastward from the vertical in which its fall began.''