Traditional wireless sensors provide point sensing and monitoring where the sensing data are coming from sensor hardwares that produce a measurable response to a physical property change. This paper introduces a new underground sensor nodes. The wireless signal strength information from networked sensors are used to produce a response to a underground condition change where the concept is called Wireless Signal Networks (WSNs). To provide practical pervasive underground sensing, we introduce Low Frequency Wireless Signal Networks (LFWSNs) which have better underground radio propagation.