Vaimãnika Shastra, meaning the Science of Aeronautics, was a text believed to be written by Subbaraya Shastry sometime between 1918-1923. Although 20th century saw some major advances in aviation technology, there are references, both on the internet and in many ancient texts, the Vaimãnika Shastra being one, about the best-kept secrets of advanced aviation technology which originated from the Indian Subcontinent.
An interesting fact that baffles the experts is the fact that the text was written based on the knowledge gathered by psychic channeling with an ancient sage, Bharadvaja. The story goes like this. Subbaraya Shastry, the author of the text, had left his home to spend the rest of his life in the forest, possibly because of a contraction of leprosy. During his time in the forest, he's believed to have spoken with the ancient sage Bharadvaja, thus channeling his knowledge of ancient flying machines.
Nine years later, an enlightened Subbaraya returned home but it was only after 16 years that he decided to spread the knowledge he had gained. Over the course of next five years, he would dictate his knowledge into Sanskrit text. A few years later, the text came into the knowledge of G.R Josyer, an honorary Director of International Academy of Sanskrit Research, Mysore, who then translated the text into English in 1973.
According to the text, Bharadwaaja defines the word Vimaana as "Vega-Saamyaat Vimaano Andajaanaam", which roughly translates to "Owing to the similarity of speed with birds, it is named Vimaana. The Vimaana is a vehicle which flies in the sky with speed comparable with birds." The publication is believed to possess 32 secrets of flying, including full-fledged air combat applications and aviation tactics. A chapter in the text 'Atha Jaatyadhikaranam' describes different varieties of Vimaanas in different yugas and also describes the mechanisms and the metals and other materials used.
NASA has been engaged in the study of mercury propulsion systems for some time now, which is mentioned in Vaimãnika Shastra. NASA is believed to have conducted a conclusive study on the descriptions and, according to their report, it has been found that many of the models face many practical challenges to fly. Time will only tell whether the descriptions in the text were true to its knowledge or if the whole episode of psychic-channelled cognition was a farce, but keeping aside its practical applications, the Vaimãnika Shastra, if true, presents another story where the Indians were far ahead in the advancement of technology.