 The tunnels running through Zhonghe-Yonghe area had to pass under narrow streets, skyscrapers and crowded blocks, with limited spaces for underground station above ground.As a result, the whole neighborhood traffic had its so-called "Dark Ages" when the Cut-and-cover method was used for station platforms, concourses and crossovers during the 1990s.Besides the river-crossing section, the work suffered from biogas below the waterfront.