arXiv:2610.33202v1 Announce Type: new Abstract: Wireless underground sensor networks (WUSNs) enable in-situ monitoring of subsurface processes and buried assets to deliver substantial social and economic benefits across diverse verticals. However, meter-scale underground monitoring remains challenging since conventional WUSNs place the entire underground device at the target sensing depth, where severe soil-induced attenuation significantly degrades communication reliability and makes wireless energy transfer (WET) inefficient and even impractical. To address this limitation, we propose Terra-Fi, a novel shallow-hub and deep-sensing architecture that decouples the sensing depth from the wireless communication and energy-replenishment depth. In Terra-Fi, a shallow wireless hub is deployed at a depth favorable for communication, WET, and maintenance, while application-specific sensing probes are extended to meter-scale target layers through power-and-signal feeders. On this basis, we first examine the depth limitations of underground communication and WET and compare representative monitoring architectures. Then, we present its system architecture, operation flow, and potential benefits. Through a precision agriculture case study, our numerical results reveal that Terra-Fi establishes more robust wireless connectivity and a longer operational lifetime than conventional fully buried WUSN architectures, particularly at greater sensing depths and higher volumetric water content. Moreover, its performance gain can be enhanced through time allocation and transmission-configuration optimization. Finally, we pinpoint practical challenges and future research directions for deploying Terra-Fi into WUSN systems.