Myself and my neighbors property have quite large swells in the ground in our sections. I never really noticed them or took much notice when I viewed the property but now I'm a little worried what they are.One looks like it has lifted up one of the Pergola legs in the past as I found one laying on top of the roof and where it is meant to go is where one of these swells are. The neighbors said they only appeared in the last 2 years as they have lived here for 20 years. This land is very sandy and the river is only about a mile or 2 away but there are quite a few streets and houses much closer to the river than ours. The swells in the ground are around 1 to 1.5 feet in height and are around 2 square metres round (this is roughly guessing, I would have to measure to get better specs). I have been digging a vege garden recently and some parts I dug were nearly 90% sand while others had a lot more soil. The mounds in the ground are only about 8 - 10 metres from the back of the house and I can even see other parts closer that could also be effected.
I also thought it could be from tree roots as there is a fairly large cedar tree and even bigger oak type of tree on the back property but these seem a bit far away (about 30 metres roughly) to be the cause.
What could I do, should I call the council, some type of specialists etc to work out what these are?