This novel mainly tells the story of a retired soldier joining an expedition team and discovering the secret base and underground airport of the Japanese army at the bottom of the lake.

The secret at the bottom of the lake in Qing Yu Nian's Taiping Courtyard was the forbidden door. According to the description in the novel, Ye Qingmei had mentioned in her letter to Fan Xian that there was a forbidden door at the bottom of the lake. It was also the final location of all secrets. Taiping Courtyard was the place where Ye Qingmei had once lived. It was also the place where she, the Qing Emperor, Chen Pingping, Fan Jian, and the others had been carefree when they were young. The secret of the Forbidden Door was related to Fan Xian and his arrival in this world. Fan Xian was a person whose spirit had " passed " into Ye Qingmei's son. He was explained as an experiment of the previous civilization. However, Fan Xian's existence violated the bottom line of human thought because his soul had " seized " the soul of a newborn child. This was a forbidden act. The secret at the bottom of the lake might involve Fan Xian's next life and the truth of Ye Qingmei's experiments and seeds. However, the specific secret content was not provided.

The secret at the bottom of the lake in Qing Yu Nian's Taiping Courtyard was the forbidden door. According to the contents of Ye Qingmei's letter to Fan Xian, there was a forbidden door hidden at the bottom of Taiping Courtyard's lake. This door was the final location of all secrets. Taiping Courtyard was the place where Ye Qingmei had once lived. It was also the place where the Qing Emperor, Chen Pingping, Fan Jian, and the others remembered when they were young. The biggest secret of the Forbidden Door was related to Fan Xian. He was someone who had a terminal illness in his previous life. His spirit " passed " into Ye Qingmei's son and became a part of this new world. However, the specific secret details were not mentioned in the information provided.

The secret at the bottom of the lake in Qing Yu Nian's Taiping Courtyard was the forbidden door. According to what Ye Qingmei mentioned in the letter, the Forbidden Door at the bottom of the lake was the final location of all secrets. Taiping Courtyard was the place where Ye Qingmei had lived for a long time. It was also the place where the Qing Emperor, Chen Pingping, Fan Jian, and the others had been carefree when they were young. It was filled with thick memories and emotions. The biggest secret of the Forbidden Door was related to Fan Xian. He was someone who had a terminal illness in his previous life. His spirit " passed " into Ye Qingmei's son and became a part of this new world. This experiment triggered a series of problems that involved the challenge of the bottom line of human thought. Fan Xian's spiritual transmigration became an important element that subverted the justice of birth, aging, illness, and death. However, the specific secret details were not mentioned in the information provided.

The secret at the bottom of the lake in Qing Yu Nian's Taiping Courtyard was the forbidden door. In her letter to Fan Xian, Ye Qingmei had mentioned that there was a forbidden door at the bottom of Taiping Courtyard's lake. This was the final location of all secrets. Taiping Courtyard was the place where Ye Qingmei lived for a long time. It was also the place where the Qing Emperor, Chen Pingping, Fan Jian, and the others remembered when they were young. The Forbidden Door was related to Fan Xian and his experience in coming to this world. Fan Xian was someone who had contracted a terminal illness in his previous life and had his spirit " transmigrated " into Ye Qingmei's son. This was explained as an experiment of the previous civilization. The Forbidden Door might be Ye Qingmei's laboratory. There might be modern technological equipment inside, such as mobile phones, navigation, computers, and so on. Opening the forbidden door might subvert order, so Ye Qingmei warned Fan Xian not to open it. Other than the Forbidden Door, the secret at the bottom of Taiping Courtyard's lake might also be related to how Ye Qingmei came to this world and her guidance to others.

The secret at the bottom of the lake in Qing Yu Nian's Taiping Courtyard was the forbidden door. According to the contents of Ye Qingmei's letter to Fan Xian, there was a forbidden door hidden at the bottom of Taiping Courtyard's lake. It was also the final location of all secrets. This Forbidden Door might be a laboratory with modern high-tech equipment, such as mobile phones, navigation, computers, etc. The Forbidden Door involved the birth, aging, sickness, and death of humans, as well as the secret of Fan Xian's arrival in this world.

The secret at the bottom of the lake in Qing Yu Nian's Taiping Courtyard was the forbidden door. In her letter to Fan Xian, Ye Qingmei mentioned that there was a forbidden door at the bottom of Taiping Courtyard's lake. It was also the final location of all secrets. The Forbidden Door was related to Fan Xian's arrival in this world. It involved the story of Fan Xian's illness in his previous life and his spirit " traveling " to Ye Qingmei's childhood. The specific secret details might need to be read in the original novel or watched in the drama version to understand.

The secret at the bottom of the lake in Qing Yu Nian's Taiping Courtyard was the forbidden door. According to Ye Qingmei's letter to Fan Xian, there was a forbidden door at the bottom of Taiping Courtyard's lake. This was the final location of all secrets. Taiping Courtyard was the place where Ye Qingmei lived for a long time. It was also the place where the Qing Emperor, Chen Pingping, Fan Jian, and the others remembered when they were young. The secret of the Forbidden Door was related to Fan Xian and his arrival in this world. Fan Xian was someone who had contracted a terminal illness in his previous life and had his spirit " transmigrated " into Ye Qingmei's son. This was explained as an experiment of the previous civilization. However, Fan Xian was the only successful experimental subject. This violated the bottom line of human thought because a spiritual energy possessing the soul of a newborn was a taboo in itself. Other than the Forbidden Door, there might be other secrets hidden at the bottom of Taiping Courtyard, such as the teleportation door or the arsenal that led to the temple. However, the specific details and content were not mentioned in the search results provided.

The secret at the bottom of the lake in Qing Yu Nian's Taiping Courtyard was the forbidden door. According to the contents of Ye Qingmei's letter to Fan Xian, there was a forbidden door hidden at the bottom of the lake. It might be a laboratory. There might be modern high-tech equipment inside, such as mobile phones, navigation, computers, and so on.

In the novels of Qing Yu Nian, the secret at the bottom of Taiping Courtyard's lake was the Forbidden Door. According to the contents of Ye Qingmei's letter to Fan Xian, there was a forbidden door hidden at the bottom of Taiping Courtyard's lake. It was also the final location of all secrets. The specific secret details were not revealed in the information provided.

In the novels of Qing Yu Nian, the secret at the bottom of Taiping Courtyard's lake was the Forbidden Door. According to the contents of Ye Qingmei's letter to Fan Xian, there was a forbidden door hidden at the bottom of Taiping Courtyard's lake. It was also the final location of all secrets. Taiping Courtyard was the place where Ye Qingmei lived for a long time. It was also the place where the Qing Emperor, Chen Pingping, Fan Jian, and the others remembered when they were young. The Forbidden Door might be a secret room that stored the Qing Emperor's books, artwork, treasures, and secret documents. This secret room symbolized the Qing Emperor's wisdom and stratagem. At the same time, it also played an important role in pushing forward the story. The specific secret details were not revealed in the information provided.