Wireless sensors in extreme environments such as underground, concrete wall, and the human body, can enable a large number of important applications. However, deploying wireless sensors in such environments on a large scale is a great challenge due to the high cost and the large profile of wireless sensors. Backscatter communications can reduce the cost and size of wireless sensors by removing most of the typical wireless components. In this paper, we propose to leverage the RFID sensors for inter-media magnetic induction-based backscatter communications (MIBC). In this way, the complexity and cost of wireless sensors can be significantly reduced. The sensors leverage magnetic signals to backscatter information which demonstrate high penetration efficiency. We design a system with channel estimation and optimal signal transmission strategy, and an optimal receiver. The channel between the aboveground reader and underground sensors are modeled by using a stratified medium model. The bit-error-rate is evaluated with different configurations. The results suggest that MIBC can be utilized for most of the inter-media applications with low power consumption and high penetration efficiency.