Because it is the closest port to mainland Britain with just 21 miles from the British port of Dover, Calais is one of the busiest cross-Channel ports on continental Europe. Regular ferry services operate from Calais to Dover (P&O and Seafrance each with over 20 crossings per day). Dunkerque could be an alternative to get over (norfolklines to Dover, same or nearly same conditions than Calais – Dover), Calais is busier though.