Please post a photo of the object's other side (which you say is flat). Also, tell us its size.
If it is non-magnetic, and it came out of the ground with no corrosion, it might be silver, or some other "white-metal" that doesn't corrode in the ground, such as White Tombac.
For now... being only able to judge by the photos of one side, it looks like a "puddle" of melted metal.
You say it is heavy for its size, so it is not melted Aluminum/Aluminium... which we diggers frquently find where a hot fire has occurred and an aluminum can got melted.