Land Subsidence in the United States
Subsidence is more common than many may realize. USGS Fact Sheet-165-00 cites several examples and causes across America.
Subsidence due to ground water pumping.
Subsidence due to sinkholes and evaporative rock.
Simpler types of vertical movement are evidenced by the age-old axiom to not place bench marks in highway side slopes.
Another thing to consider would be to ask yourself why NGS adopted the NGS 3-D Monument