Abstract. If the QCD axion solves the strong CP problem then light axion-like-particles (ALPs) are expected to be ubiquitous in string theory (the string axiverse) and give rise to axion cosmic strings, in which compact dimensions seem to decompactify at the origin. In this talk I review cosmic strings, axions and the string axiverse, before turning to study the properties of axiverse cosmic strings in a toroidal compactification, including the crucial effects of moduli stabilization. At the core, the cosmic string is revealed to be a brane in disguise, and at long distances there are potential new signatures involving variations in Standard Model parameters and equivalence principle violations. We find indications of similar behaviour in more realistic Calabi-Yau manifolds.