 The loop itself still exists , although it was penetrated by a bomb and flooded during the Blitz in the Second World War . Fortunately , the loop had been sealed off years before . In September 1938 , during the Sudeten Crisis , when war appeared imminent , the Bakerloo and Northern line tunnels at Embankment were temporarily sealed with concrete to protect against flooding through bombing . The blockage was removed after little more than a week once the crisis had passed . At the outbreak of World War II in September 1939 , the tunnels were blocked again until electrically powered emergency doors could be installed in the tunnel mouths . The tunnels reopened in December 1939 . 
