During the excavation of underground tunnels and ore mining, accidents related to sudden water inflows often occur. As the only technique used for the direct detection of groundwater, nuclear magnetic resonance (NMR) has advantages for the detection of disaster-inducing water flows. However, NMR has commonly been applied only at the ground surface using large-size loop with several turns (typically tens of meters). For the first time, we demonstrate that the water signal in a tunnel can be directly detected using an underground nuclear magnetic resonance (UNMR) experiment. Specifically, we describe the design of a six-meter multi-turn transmitting coil and receiving coil. By conducting UNMR measurements in the Dadushan Tunnel, located in southwestern China, we not only verified the ability to detect the UNMR signal, but also matched the observed data using a 1D inverse model.