When Colin drops the rock to see how deep the fissure is, he counts four bananas, which one assumes is four seconds. He then says that this means the distance to the bottom where the rock splashed into the water is one hundred and thirty feet. According to the standard kinematic equation for calculating height from how long an object drops, which formula is x = v_0 t + 1/2 at^2, where x is the distance, v_0 is the initial velocity, which is zero, since Colin dropped the rock, rather than throwing it down, a = acceleration due to gravity, which is 32 feet per second per second, and t equals time. Let t = 4, then one gets : x (distance downwards) = 1/2 times 32 times 4^2, and this equals 16 times 16, which is 256 feet. In addition, if the stone hit the water 256 feet down, at a final velocity of 128 feet per second, the time the sound could travel back up to Colin, where the speed of sound in air is 1125 feet per second is just over a quarter of a second. If this is taken off, then the time of the stone falling is approximately 3.75 seconds, making the distance 225 feet, but this once more has its own time delay in hearing the stone hit the water - but let one say, give or take, that the depth of the fissure is at least two hundred feet, and not 130.