Zhong Ma was a fortress used by the Japanese prior to World War II. It was constructed in Manchuria after the Japanese invasion in 1931 and 1932, mainly by enslaved peasants in the region. The Japanese used Zhong Ma to conduct experiments in biological warfare. These experiments were led by the young military scientist Ishii Shiro and sponsored by the Japanese Emperor himself.
In 1933, Mai Ling was a captive in Zhong Ma. The Seventh Doctor and Edward Grainger were briefly imprisoned there when they arrived to rescue Mai Ling. While there, Edward was deliberately infected with bubonic plague, which evolved into pneumonic plague, and samples of the Doctor's blood were taken. The three were able to escape after the Doctor destroyed his blood samples, and Edward was cured.
The experiments conducted in Zhong Ma were later continued by Unit 731. Germ-ridden packages were dropped on towns and villages throughout the region during the war, killing hundreds of thousands of people.
According to the Doctor, the people involved with the experiments would never be called to account for their crimes after Japan surrendered to the United States in 1945. The US went as far as to use the data gathered from the research for their own purposes. (PROSE: Log 384)