In Berlin, from 1961 until the wall came down in 1989, more than 5,000 East Germans managed to cross the border, a number of them through the sewers system.
While the water supply was disconnected between both part of the city since 1950, the sewers remained city wide for a long time but secured by grills, electric systems, etc.
During this time an extensive program of division of the sewer system between both parts of the city was started with new sewage pumping stations and pressure sewer lines.
Several escape attempts through the sewers were repeatedly until 1980.
Read more (in German) at the Berlin Waterworks Museum