Dear teachers,
"At no period in human history has there been a population explosion of the magnitude as the one in the twenty century."
Could you please explain why "has" was placed before "there"? Is it an inversion? Why using "inversion" in this case?
Original Post
"... in the twentieth century," Coco.

Now, here's what Michael Swan* has to say about such an inversion as you've cited:

"If a negative adverb or adverbial expression is put at the beginning of a clause for emphasis, it is usually followed by auxiliary verb + subject. These structures are mostly rather formal.

Under no circumstances can we cash cheques.
At no time was the President aware of what was happening.
Not until much later did she learn who her real father was.

"The same structure is possible after a complete clause beginning not until ...

Not until he received her letter did he fully understand her feelings.

"Inversion is also used after restrictive words like hardly, seldom, rarely, little, and never, and after only + time expression. This is formal or literary.

Hardly had I arrived when trouble started.
Seldom have I seen such a remarkable creature.
Little did he realize the danger he faced.

I hope this helps, Coco.

*Michael Swan. Practical English Usage. Oxford University Press. 2005

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