I was fishing a small lake yesterday where I have not seen a depth below 12 feet.
Went to a location the sonar logs showed several nearby spots with high probability of fish.
My display seemed to go crazy here showing the depth at over 300 feet.
When set to manual with 10 foot range I would see what I expected, put back on aut and see the same thing.
Put unit into and out of standby, same thing,
Turned unit off and back on, same thing.
When moved a bit aways came back to normal.
Gruessing this might be a place where the spring feeds the lake, the bottom is thin and the sonar goes through it measuring the depth of the water chamber below the lake.
Does this make sense?
Any other ideas?

crashed alien spaceship... lol J/K I think you are on to something with the spring theory - pretty cool that it can detect something like that if in fact that is what it turns out to be.