Answer:
The most probable and logical possibility would be that the groundwater beneath Nelson's house has been exhausted.
Step-by-step explanation:
If there is a groundwater bed lying below the foundation of the house, it is necessary for the groundwater to stay intact in its place because if it is extracted from beneath the foundation of the house, the vacant space left after the water gets exhausted would create a cavity which might shrink due to the weight of the house structure above it and may cause the house to sag an sink.