I'm currently in my third year of an undergraduate degree, mainly doing courses in abstract algebra. I'm enjoying my degree, but every so often when I'm swamped with difficult proofs, I feel like I need some extra inspiration to give myself more mathematical energy and remind me why I got interested in maths in the first place - otherwise I wear myself out. I do things like watching videos on numberphile, but I feel like this is aimed at a much lower level than I'm at.
My question: What is a good resource that features advanced mathematics and isn't shy about going quite deep, but is still fairly recreational in its nature? I don't really care what form this resource takes, it could be blogs, books, videos etc.
Any help would be greatly appreciated.