Nissan Sentra Plug on Bottom of Engine?

I have a 2000 Nissan Sentra with a 1.8L I4 and a manual 5-speed. There is a plug for what looks to be some kind of sensor on the very bottom of the engine. It has broken off (likely by running over something) and is dangling there. The check engine light is on, but I don't know what to call this part to even try to get a replacement. I don't think this is the crankshaft position sensor. It's in the wrong place.