Many national cemeteries have a section dedicated to unknown and missing soldiers and those have been cremated or made donations to science.
The marker states:
"The Markers In This Memorial Area Honor Veterans Whose Remains Have Not Been Recovered Or Identified, Were Buried At Sea, Donated To Science, Or Cremated And The Ashes Scattered."
The common maker for unknown soldiers:
The above photos are from Winchester National Cemetery, Winchester City, West Virginia.
To all military members whose final journey is known only to God, thank you for your sacrifice.