Columbia subject headings:
Harvard subject headings:
Nord-Pas-de-Calais (France)--Description and travel.
Underground areas--France--Nord-Pas-de-Calais.
Princeton subject headings:
Souterrains--France--Pas-de-Calais.
Tunnels--France--Pas-de-Calais.
Underground architecture--France--Pas-de-Calais.
Underground areas--France--Pas-de-Calais.
Underground construction.