Given that we are trying to map the Milky Way from the inside, the fact that we've deduced as much as we have is pretty amazing. Thinking about that mapping process and the use of astronomy in all part of the electromagnetic spectrum from radio to X-ray and gamma ray, explain which aspect of the Galaxy's structure do think is our most interesting - or hardest to understand - discovery? Give one example of astronomical observations that have aided astronomers in understand the Milky Way's size and structure.