On this day, 56 years ago, September 23, 1969 (August 12, 1969 in the lunar calendar), China successfully conducted its first underground nuclear test. Underground nuclear tests are divided into flat holes and vertical shafts. China originally scheduled to conduct the first flat hole test on May 9, 1960. However, in order to obtain a practical missile warhead faster and concentrate on developing hydrogen bombs, this test was changed to an atmospheric test to measure the power and performance of the warhead. The first flat hole nuclear test was not conducted until 00:15 on September 23, 1969. The second flat-hole nuclear test on December 17, 1978 obtained a large amount of important data and contributed a lot to subsequent underground nuclear tests. The third flat-hole nuclear test was first carried out in granite on October 17, 1976. The fifth flat-hole nuclear test on December 19, 1984 was the first principle experiment of a neutron bomb. This verification bomb was a complete success and laid the foundation for a comprehensive breakthrough in the bottleneck of neutron bomb technology and the first nuclear test of neutron bombs. It also refuted the United States 'accusation that China "stole" the secrets of the neutron bomb. In April 1967, China selected Singerr, northwest of Lop Nur, as the shaft nuclear test site. This method is the main type of underground nuclear testing. The first granite shaft with a depth of 300 meters and a diameter of 2.5 meters was drilled in April 1975, but the first shaft nuclear test was not successful until 09:00 on October 14, 1978. After the atmospheric nuclear test on October 16, 1980, subsequent nuclear tests in China went underground; including neutron bombs and recent nuclear tests, shaft methods were also used. In the 1960s and 1970s, due to insufficient understanding of scientific laws, China failed completely or partially in three nuclear tests, but the proportion was not high. Starting from November 2, 1964, foreign countries began to record Lop Nur earthquake data. Specific waveforms can be obtained from earthquake records such as the Global Standardized Seismic Research Network and the Joint Earthquake Research Institute. Some seismic stations regularly report to the National Earthquake Information Center and the United States International Earthquake Center. In the mid-1990s, the United States used a computer program to automatically access the database of the National Seismic Information Center three times a day to monitor China's nuclear tests. Since October 3, 1972, the West has used commercial satellites to reconnoiter the Lop Nur nuclear test site, using multi-spectral scanning, reflected beam vidicon, terrain mapper, high-resolution cameras, infrared reconnaissance instruments, space shuttle synthetic aperture radar, etc. Astronauts also conduct manual photography. Lop Nur is about 2300 kilometers west of the north. Four locations have been selected, west of Dunhuang in Gansu and north of Lop Nur in Xinjiang. Experts from China and Russia believe that the area 160 kilometers west of Dunhuang is the best. The nuclear test site is divided into three areas. Malan Science City is located to the northwest of the test site, near the intersection of the two trunk highways is the main command post and residential area for technicians. There is a secret research institute, namely the nuclear test center, dozens of kilometers northwest of Malan. It is surrounded by mountains on three sides and there is an external road. The nuclear test was carried out in the desert not far from Malan to the southeast of the test site. The Central Division is between the Malan Division and the Southeast Division. It is an underground nuclear test site, but several low-yield ground nuclear tests were also conducted in the early years. The central zone is divided into three underground nuclear test areas; The southern test area is in the mountains; the first two underground nuclear tests on September 22, 1969 and October 27, 1975 were conducted here. These two nuclear tests tested the containment technology of underground nuclear explosions. After these two tests, the Pingdong nuclear test was transferred to the Western Test Area. The nuclear tests in this test area were all low-yield. During the test, the test data of seismic weapons and the safety and rationality of the nuclear test site were also tested. The principle test and first explosion test of the neutron bomb were carried out here. The eastern test area is a low-slope land between the Western Hills, where deep shaft underground nuclear tests are carried out. Many nuclear tests have been conducted here in recent years, including the Dongfeng-41 warhead test. As of the end of May 1998, a total of 2058 nuclear tests had been conducted around the world, of which 1032 were conducted by the United States, 715 by the former Soviet Union, 210 by France, 45 by the United Kingdom, 45 by China, 5 by India, and 6 by Pakistan