The visitor from another world (part two)

'Wait, then how come everything is still here then?'

"But how is it possible to hide something this big in a mere hundred years?" It didn't make sense. 

There were 3.2 billion people on earth and that was a lot of people!

"Well, about that." The headmaster chuckled. "It happened a few thousand years ago and someone changed the space-time and history itself. In the original timeline, the world was ruined and humanity was almost extinct. Only a handful of people survived- the strongest of the strongest- and came back to the past to stop the invasion from the first place."

Sounded very complicated.

"And you know that because?" Because he sure didn't seem like a hundred years old man to me. 

He chuckled. "I'm one of them."

He still didn't look a hundred years old. 

"And the invaders were from Neil? But wait, something doesn't make sense."