It seems like every few months there's a new discovery of an old site under newer construction. Some times the old sites are fairly old, some times they're from the 1800s. How exactly do these sites end up so far under ground that new construction is made on top of them without people realizing? As an example, how do parts of a house built in the 1700s sink below the level of the rest of the land and what exactly leads to these parts being covered?