Years ago a solid floor would be of brick or quarry tiles laid directly onto the earth or perhaps a bed of sand or ash, and without any form of damp proofing.
Over the years these floors become damp and uneven. Covering them with a carpet or floor vinyl adds to the problem.
If they remain fairly level they can often be coated with mastic asphalt which will provide a level surface and effective damp proof membrane.
If they are particularly uneven they will probably have to be excavated and relaid with a concrete screed on a suitable damp proof membrane.