Field-Programmable Gate Arrays (FPGAs) are integrated circuits that implement reconfigurable hardware. They are used in modern systems, creating specialized, highly-optimized integrated circuits without the need to design and manufacture dedicated chips. As the capacity of FPGAs grows, it is increasingly common for designers to incorporate implementations of algorithms and protocols from a range of third-party sources. The monolithic nature of FPGAs means that all on-chip circuits, including third party black-box designs, must share common on-chip infrastructure, such as routing resources. In this paper, we observe that a "long" routing wire carrying a logical 1 reduces the propagation delay of other adjacent but unconnected long wires in the FPGA interconnect, thereby leaking information about its state. We exploit this effect and propose a communication channel that can be used for both covert transmissions between circuits, and for exfiltration of secrets from the chip. We show that the effect is measurable for both static and dynamic signals, and that it can be detected using very small on-board circuits. In our prototype, we are able to correctly infer the logical state of an adjacent long wire over 99% of the time, even without error correction, and for signals that are maintained for as little as 82us. Using a Manchester encoding scheme, our channel bandwidth is as high as 6kbps. We characterize the channel in detail and show that it is measurable even when multiple competing circuits are present and can be replicated on different generations and families of Xilinx devices (Virtex 5, Virtex 6, and Artix 7). Finally, we propose countermeasures that can be deployed by systems and tools designers to reduce the impact of this information leakage.