Topological materials have unique electronic transport properties1. They can, for example, be insulating in the bulk, yet present conducting states at their surfaces, as though they were coated by a thin layer of metal. Electronic conduction through these conducting states is also not entirely conventional. In some materials, surface currents are protected against disturbances introduced by defects, and are able to turn around sharp corners without experiencing scattering. Topology also provides a concise and elegant theoretical framework to explain a range of physical phenomena ranging from the quantum Hall effect to the unusual electrical conductivity of bismuth2. Although research in topological materials still falls squarely in the realm of fundamental physics, the unique properties of topological insulators have prompted speculation that they could be key components for a variety of novel electronic devices. Examples of these have included single-mode lasers3, robust waveguides4 and circulators5. Despite this progress, the direct experimental determination of the topological properties of materials remains an open challenge. Now, writing in Nature Materials, Zhi-Kang Lin and colleagues report experimental signatures of topology by introducing crystalline defects in acoustic metamaterials6.