Scientists have just discovered a steel eating bacteria that is eating at the Titanic. My theory: this bacteria comes every few thousand years brought about by the geomagnetic reversal. The Earth recycles itself... causing humans to go into wars because of loss of resources. Massive structures get destroyed... by either environmental or human damage leaving remnants behind. Meanwhile, this steel eating bacteria destroys all evidence of intelligent civilization so that humans have to begin all over again. Structures like the Pyramids remain because theyre made of stone. same goes for stonehenge and the caves. Modern human leaves no remnants. No evidence of their superiority. Man begins again. That explains why scientists have found hidden cities underwater that are 10s of thousands of years older than we thought could be possible.