Harbin metro testing in August 2012

Harbin

subway

Harbin's 17.5 km underground subway opened in September 2013. It is expected to be the first line of a 10 line, 340 km system. But there were traces of a "mystery subway" long before this real one opened.
 
According to 1997 Jane's World Railways, Harbin (formerly known as Ping Kiang) has a 5.4 mile metro. No one who has lived in Harbin who I spoke with appeared to know of it's existence. An unfinished 9 km metro line was also
been reported by Metropla.net. This would be in line with an article which reports a line to begin construction in 2003 which is based on "the original 7381 civil air-defense project of 10.1 km."  The report said the 24.6 km line could be opened as early as 2005.

Here, plans for a 17.3 km, 15 station light rail (elevated metro in western terms) 
were shown, but not much else. Digital Harbin also presented a feature about an "existing 10.1 km, 5 station existing tunnel",  and a submitted proposal to  develop it into a 24.6 km, 24 station subway, including 18 stations for the 17.5 km main line (suspiciously similar to the light rail proposal) and 6 stations for the 7.1 km subsidiary line.

Metropolitan Railway Department
Harbin Municipal Council
City Administration Building
Harbin, China

Finally, in December 2006, ground was broken on a 20 station, 14.4 km metro. The route is said to follow the emergency evacuation tunnels left over from WWII. This Youtube video shows the subway construction and the old tunnels.

Harbin is also known for the notorious area 731.