An explosive at ground level leaves a crater with a diameter that is proportional to the energy of the explosion raised to the (1/3) power; an explosion of 1 megaton of TNT leaves a crater with a 1 km diameter. Below Lake Huron in Michigan there is an ancient impact crater with a 50 km diameter. What was the kinetic energy associated with that impact, in terms of megaton of TNT. Note: 1 megaton of TNT yields 4.2 x 1015J.