He mentioned the "exact antipode on Earth", so he's probably thinking of an effect similar to antipodal chaotic terrain [1]. The fact that sound was only audibl

He mentioned the "exact antipode on Earth", so he's probably thinking of an effect similar to antipodal chaotic terrain [1]. The fact that sound was only audible up to 3000 miles in radius is not enough to rule it out, you need other arguments. [1] http://en.wikipedia.org/wiki/Caloris_Basin#Antipodal_chaotic_terrain_and_global_effects http://en.wikipedia.org/wiki/Caloris_Basin#Antipodal_chaotic... ("It is thought by some to have been created as seismic waves from the impact converged on the opposite side of the planet.")