Sub-sonics
I learn a lot about subsonics from chatting with others about the volcano in Iceland last year. The way that gas bubbles move through the magma during eruption gives out noise, both acoustic and seismic, and can be quite diagnostic to the eruptive phase. Typically, these signals are down between 1 and 10 Hz. Fortunately, Iceland had seismic stations nearby that were able to filter the sounds in a few different bands. Using that data, it was possible to detect the eruptions before there was visible lava or on foggy days. People on the chat got really good at picking out the patterns.
I have seen that much research is being done to use sensors to detect tornados using similar techniques.