Graham's Number, a number so large the observable universe couldn't hypothetically be adapted to contain it, was devised by mathematician Ronald Graham to explain the upper bound of a problem he was working on. Rather than just being inscrutably gigantic and only describable through notation, it's possible to calculate the last digits (but not the first).
The Graham's Number Clock highlights the viewer's current hour, minute, and second values within the last 500 digits of Graham's Number. That's it... for now.
Background image from NASA / Hubble.