The origin of dampness in this case is wet earth that is in contact with submerged walls, either because it has rained or because it is in an area where natural groundwater is present.
If walls that are in contact with wet earth are not waterproofed well enough, or have deteriorated over time, water begins to seep into the interior of the house. The wetter the soil, and the less waterproofed and more porous the walls, the greater the amount of dampness in the house – these factors are all directly related.