It's impressive, yes. And probably we don't know exactly how it was done. But that's not the same as saying it's an unexplained mystery - we just don't know which particular methods they used, even if we know one or more ways in which it could have been done.
> And probably we don't know exactly how it was done. But that's not the same as saying it's an unexplained mystery
That is the meaning of unexplained mystery, something mysterious and unexplained.
"(W)e don't know exactly how it was done" is exactly what "unexplained mystery" means.
As opposed to explained mysteries? /s
An explained mystery would be something for which the method would be non-obvious, but nonetheless the method was known. Like a magic trick the solution of which were pubic knowledge - mysterious but explained in sources in the public domain.
So what's something where we can come up with several plausible methods, but don't have the evidence to conclusively decide which method actually was used?