Source
This question came up in the discussion between Kevin BuzzardKevin Buzzard and Minhyong KimMinhyong Kim in the comments to Smooth proper scheme over ZSmooth proper scheme over Z. It was 2 weeks ago, so I took the liberty of posting it as community wiki.
Question
Is there an example of smooth proper variety $X \to \mathop{\text{Spec}}\mathbb Z$ such that $\pi_1(X) \ne 0$?
About tags
We recently had other questions of the form "Example of ... with everywhere good reduction at $\mathbb Z$" (local-globallocal-global, abelian varietiesabelian varieties). I think it would be interesting to create a tag to group these. Thoughts?