The Hypogeum of Malta is an underground temple discovered in 1920.  Believed to be constructed 6000 years ago, it is the second oldest man made structure on earth. The oldest being Gobekli Tepe.  The Hypogeum is well worth a ponder – what makes it so mind blowing is that it seems to have been designed to amplify the voice, but only at certain megahertz. Three stories deep, when speaking in the “Oracle Room” the human voice is amplified through out the entire structure. But only if it’s a male voice, female voices don’t have the right frequency.