I have read a lot of articles about what happens if you die without a will in a variety of situations (married, single, domestic partners, with/without children, etc.). Most "extreme" is that you die single without a will:
If you are single and childless, your parents will receive your entire estate if they are both living. Otherwise it will be divided among your siblings (including half-siblings) and your surviving parent, if one parent has already died.
If you have no surviving parents at the time of your death, then your entire estate will be divided among siblings, in equal parts.
If there are no surviving parents, siblings, or descendant’s of siblings (nieces and nephews), then the relatives on your mother’s side would inherit one-half of the estate, with the other one-half passing to the relatives on your father’s side.
But what happens if they can't locate any relatives (i.e. your parents immigrated to the US and you don't know your relatives)? What happens to your assets?