In fact, there is indeed an underground palace under the Big Wild Goose Pagoda. Of course, this is not well documented in history. However, it is rumored that there is indeed an underground palace under the Big Wild Goose Pagoda, which was designed and built by Tang Xuanzang at that time. This is mainly because after learning Buddhist scriptures from the west, Tang Xuanzang brought back many Buddha statues and bibles from India, and he put them in the underground palace of the Big Wild Goose Pagoda built at that time.
However, according to the historical development, there should be. After all, the preserved Buddha statues and bibles that Tang Xuanzang brought back from India are basically not stored anywhere else. At this time, the underground palace built under Xi 'an Big Wild Goose Pagoda is probably used to store these things. Of course, the value is also very expensive.
I believe it is precisely because of the protection of cultural relics and its high value that the underground palace under the Big Wild Goose Pagoda has been blocked and now it is not allowed to visit. But according to the development of history, it does exist, and you can read different books to verify it accordingly, so that you can get some different opinions.