Riemannian Manifolds with holonomy G_2 are interesting both for geometers and for theoretical physicists. I will give a short introduction into the basics of G_2-geometry. I will then introduce the Crowley-Nordström v-invariant and describe the extra twisted sum construction. As a result, we will see that the moduli space of metrics with holonomy G−​2 is disconnected for some closed 7-manifolds.