___ is known to all,Columbus used a ship to discover America about 450 years ago.

it is known to all 众所周知
as we all know 正如我们所知道的那样It is known to all 后面不是应该接从句吗As is known to all属于放句首的定语从句，也可理解为状语
It is known to all后接从句that
As is known to all,the earth travels around the sun.
It is known to all （of us） that all matter is made up of molecules and that molecules are always moving.

这样看来，这里应该填As。