It was, at least from mankind’s perspective, the most important cosmic collision since the creation of the Universe.

Four-and-a-half billion years ago, an asteroid from deep space — nobody knows from where, because cosmic milestones had not yet formed — crashed into the still-evolving Earth and chipped off a chunk of it.

The chunk later became our old familiar Moon. Thank you, Theia.

(Oh, by the way, the asteroid is named Theia.)

So, we have our moon, but where is Theia? Where did she go?

This question has puzzled scientists for decades, but in recent years some scientific sleuths have tracked her down.

That is what a research team led by Qian Yuan, a scientist with Arizona State University, believes. Now, the inchoate Earth of 4.5 billion years ago — the proto-Earth — was yet to develop a solid crust. An object plunging into it at great speed would be like bunging a peanut into a bowl of pudding — where could the peanut go but inside the pudding.

The impact ripped apart a chunk of the Earth and created a cloud of rocks, which later coalesced as the moon. But the impactor herself just went in.

What gave Theia away were two large chunks sitting on top of the Earth’s core, in the mantle, that behaved somewhat differently — seismic waves passed through them slower. Seismologists call these as ‘large, low-shear velocity provinces’ or LLSVPs.

Like warts on the Earth’s core, they are mountains that are a hundred times taller than Mount Everest. One is under Africa and the other under the Pacific Ocean.

Of course, all this is still theory, even if held together by specs of data generated by simulations of Theia and isotopic analysis of moon rocks. The theory could gain support if rocks from the moon’s crater in its south pole are brought to Earth for analysis — these rocks are an undisturbed part of the moon, so they could reveal the deep past.

Still, Yuan’s theory brings up echoes of an old philosophy: What you look for all over could very well be within you.