Amsterdam is constructing a new metro traject, the Noord/Zuidlijn. As you might know, Amsterdam is build in and on water on reclaimed land: polders . So underground you find, instead of sand or clay or rocks, a swampy, wet-like sort of soil. Not easy to dig metro tunnels in, but still they manage.
You can visit the works and see underground how the metro tunnels are built at the Uitkijkpunt, at the Rokin.
Every day except Sunday, from 13.00 to 18.00 hours. It’s free.