Not abandoned and limited days if the week to see it (I’ve missed this every time I’ve been through by a day) but Carriere De Montigny is an underground hospital from WWI near the French Belge border. Often called Suzanne.
http://www.musee-t...rieres-de-Montigny Italy and France both have plenty to check out. Rich industrial history in the more northern parts of each. My advice for all places in Europe - like here - take the old roads. Maybe map your sleeping places and then fan out from there. These two things have left me most successful when euro-road-tripping