...and almost always is, except in the trivial cases of tiny numbers right near the start. Which is why it doesn't actually work as an encryption algorithm.
The other idea I've always liked is a length that, to enough precision, encodes something in the decimal places. Then in theory you could just cut something, say a diamond, to this impossible precision, and then measure it later. I assume that, like the Pi example, this is impractical not only in practice but even in theory, even if we had impossibly-accurate cutters and measurers.