If we build a vacuum tower 10,000 feet high and we dropped 1 million ton of weight into the vacuum tower, this will create an explosive power equivalent to half the power of the atomic bomb dropped in Hiroshima.
Since there's no terminal velocity and since there's no drag in a vacuum, the speed will keep on accelerating till you reach near the speed of light!!
The destructive effect depends on mass because kinetic energy which is spent in the moment of "braking" during the impact depends on mass T=1/2 mv^2. When you calculate this you get that T is approx 30TJoules which is one-hour energy output of 10 large power plants!!
In comparison, the atom bomb dropped in Hiroshima is about 63TJoules.