In eastern Ontario, Canada, there's a place where, when walking and stomping, you can hear a hollowness in the ground.
The area is primarily igneous rock with many large boulders, and the spot in question is directly over a very large buried boulder.
The boulder is under 15cm of gravel and 30cm of sand.
There are caves in this area.
Is the hollow sound an indication of a cavern/caves, sinkhole, geode or something else?