Helsinki has spent decades building beneath itself, turning excavated granite spaces into swimming pools, sports halls, playgrounds, parking, utilities and even an art gallery.

One underground swimming complex welcomes hundreds of thousands of visitors each year and can also serve as a civil-defense shelter.

Why it’s worth going

This is infrastructure tourism at its best: you can actually use the spaces rather than just tour them.

When not to

Some underground facilities have limited public access or serve dual emergency functions, so check opening hours and whether a site is open to visitors before planning around it.