Abstract: We present the design, implementation, and evaluation of U2B, a technology that enables ultra-wideband backscatter in underwater environments. At the core of U2B’s design is a novel metamaterialinspired transducer for underwater backscatter, and algorithms that enable self-interference cancellation and FDMA-based medium access control.We fabricated U2B nodes and tested them in a river across different weather conditions, including snow and rain. Our empirical evaluation demonstrates that U2B can achieve throughputs up to 20 kbps, an operational range up to 62 m, and can scale to networks with more than 10 nodes. In comparison to the state-of-the-art system for underwater backscatter, our design achieves 5x more throughput and 6x more communication range. Moreover, our evaluation represents the first experimental validation of underwater backscatter in the wild.