How Did Arrow Get A Kryptonite Arrow in "Crisis on Earth-X"?

The CW's 'Crisis on Earth-X' crossover kicked off with some major revelations and huge fights for [...]

The CW's "Crisis on Earth-X" crossover kicked off with some major revelations and huge fights for the DC Comics superheroes, and while there are loads of questions that beg to be answered, one sticks out among the rest.

How the heck did Arrow get his hands on Kryptonite?

In the crossover, Green Arrow, The Flash, and Supergirl come face to face with their Earth-X counterparts after the Nazi versions steal a prism to re-create a Red Sun. After revealing their faces and showing they're the evil versions of Kara Danvers and Oliver Queen (accompanied by Eobard Thawne to counter The Flash).

After the revelation, the good Oliver retrieves an arrow from his quiver and fires it at Overgirl. The arrow unshields its lead casing in mid trajectory, revealing a bright green Kryptonite arrowhead that drives right into Overgirl's shoulder.

A shocked Supergirl asks, "Why do you have a Kryptonite arrow?"

Arrow responds, "In case an evil you ever showed up!"

The episode doesn't go into further detail than that, possibly due to a fight erupting and a building falling down.

But how would Oliver get his hands on Kryptonite? It's an interesting question that should be explored in future episodes, though based on how quickly it was forgotten in "Crisis On Earth-X, Part 2" makes us think it's all said and done with.

Kryptonians don't exist on Earth-1 — none that we know of, anyways. So Oliver's knowledge of the beings basically only extends to his experiences with Supergirl. And since she's from a different Earth that Oliver doesn't frequently visit, it's not like he had ample opportunities to retrieve some.

Did Krypton still explode in the Earth-1 Universe? But without any survivors — or Earth-dwelling survivors? And did those chunks of the planet make it to Star City? Or did he find out Kara's weaknesses and scavenge her reality in order to find the only object that could hurt her?

It was a fun comic book-y moment, but one that creates a major plot hole in the Arrowverse.

"Crisis on Earth-X" concludes tonight with episodes of CW's The Flash and Legends of Tomorrow.