There is no credible scientific evidence to support the claim that Admiral Richard E. Byrd encountered a lost civilization during his expeditions to the poles or witnessed the existence of a hidden underground base with extinct animals and plants in his secret journal entries. The veracity of his entries themselves have been called into question, with some disputing that they were even written by Byrd in the first place.
Moreover, the notion of mammoths or similar creatures living underground is not supported by any credible scientific evidence. While it is true that there are many species living in underground or cave environments, most of these species are adapted to their subterranean habitats and have evolved specific physiological and behavioral traits to survive in this environment.
On the subject of the Hollow Earth theory, this is a popular but unsupported belief that suggests that the Earth is not a solid sphere, but instead has a hollow interior with entrances at the poles. Advocates of the Hollow Earth theory suggest that there may be entire civilizations living inside the Earth, separated from the rest of humanity.
However, this theory has been thoroughly debunked by science, as there is no scientific evidence to support the idea of a hollow Earth capable of supporting life. The laws of physics and geology make it highly unlikely that the Earth is hollow, and any such large-scale structure would be extremely unstable and would have long ago collapsed under its own weight.
In summary, it's essential to critically evaluate any claim or theory through the lens of scientific evidence and rigor. Claims of lost civilizations or hidden underground bases should be supported by credible evidence, and unsupported theories such as the Hollow Earth concept should be approached with a great deal of skepticism.