The phenomenon you're describing, where sounds appear to come from underground even though there is no apparent source, is known as "mysterious sounds" or "earth sounds." These occurrences have been reported in various parts of the world and have given rise to different explanations and theories.
One common explanation for mysterious sounds is the movement of tectonic plates beneath the Earth's surface. Tectonic activity, such as the shifting and grinding of plates, can generate low-frequency vibrations that may be perceived as rumbling or humming sounds. These sounds can travel through the Earth and be heard at the surface, even though there is no visible source.
Another factor that can contribute to underground sounds is the movement of fluids and gases within the Earth. For example, the movement of groundwater, particularly in underground aquifers or caves, can create sounds as it flows through narrow passages or encounters obstacles. Similarly, the movement of gases, such as trapped air pockets or escaping gases from geothermal activity, can produce audible sounds.
In some cases, the acoustic properties of the surrounding environment, such as the composition of the soil or rock, can amplify or transmit sounds in unusual ways, making them seem like they are coming from underground.
It's important to note that while mysterious sounds do occur, they are often difficult to pinpoint and study. Various factors can contribute to these sounds, and their specific origins can be challenging to determine without detailed scientific investigation and monitoring.