Time Travel

[Robert Vermaat]

Put a granite market in the ground that says "here was the site of x" and it'll last a very long time.

That's exactly what they did in a children's novel written about 40 years ago. A teenager goes back in time (by accident) and can't be retrieved immediately, and ends up in the Children's Crusade. The scientists back home then get word to him that they left a marker in Flornce (I think it was), plus a date and time when he has to be there.