While a disaster has certainly destroyed land records from the 18th to 20th century in many states and counties, a record that confirms an "alternative" chain of title is still able to be secured by using alternative public databases (e.g. municipal and assessment records), court proceedings (e.g. probate and judicial cases), and secondary archives to bypass the destroyed registry books. These records can be used to show someone held title to the land.
Alternatively, a court motion such as a Quiet Title Action, or less likely an Adverse Possession Action, can grant full possession of the land legally, thus "bypassing" the "unknown owner" in the Chain of Title. This process can be used to "seal the gap" in records and bring forth the land patent as normal. In fact, these actions mirror the land patent process itself, such as posting notices locally and in newspapers for a period of time, when no reply occurs the full title transfers back to you. In short, this is how one can legally cover gaps in history, if the land had "owners unknown", and more!
NOTE: The Quiet Title or Adverse Possession Action must have a mortgaged property go into the name of the mortgagor(s) or a Due In Full / Balloon Payment clause will come into effect potentially.