It's not an artifact exactly, but the Happisburgh footprints, discovered not far from me, gave me a visceral feeling of just how deep human time is. They are a set of fossilized hominid footprints made in the Norfolk mud 800,000 years ago. In the UK, there's old stuff all over the place (I live in a house that was built in the late 17th Century) so you get used to it, but 800,000 years is a length of time I find it hard to conceive of.

https://en.wikipedia.org/wiki/Happisburgh_footprints