In popular culture, mathematics are often split between applied mathematics, which provide tools for science and engineering, and pure mathematics, that are somewhat useless. Although this distinction is dubious, this might be a real issue when searching for funding. This distinction is often replied with the insight that "pure math" find their applications decades, or centuries afterwards.
And yet, there has to be examples where pure maths stayed pure and without applications until now, and we can't really foresee how they could be useful for anything other than themselves. What are some of the oldest or most iconic examples of this ?