Heaven at SETI's Doorstep Page 4

  * On November 20th, 1969, the Apollo 12 crew jettisoned the lunar module ascent stage causing it to crash onto the moon. The Lunar Module’s impact (about 40 miles from the Apollo 12 landing site) set off an artificial moonquake of startling characteristics - the moon reverberated like a bell for more than an hour. This phenomenon was repeated with Apollo 13 (intentionally commanding the third stage to impact on to the moon), with even more startling results. Seismic instruments recorded that the reverberations lasted for three hours and twenty minutes and traveled downward to a depth of twenty-five miles, leading to the conclusion that the moon has an unusually light core or even no-core at all.