12 inches isn’t that bad as far as subsidence issues can get. The ground below you is filled with very tiny holes and gaps that water seeps down into and fills and flows through, kind of like a sponge. While water fills those pores it’s difficult to squish down the soil.

If a heavy weight is on them like a building, though, that can put on enough pressure to push the water out of those pores and flow down out and around. A mix of soil and voids/air give much less resistance to weight than a mix of soil and water filling the gaps so then the voids get compacted down and the ground sinks. Different soils in different conditions may not evenly subside and that can give you a situation like the Leaning Tower of Pisa where one side was subsiding faster than the other side. In modern construction people try to get the water out and compact the ground ahead of time so that it acts more consistently and doesn’t make big changes when a heavy object is placed on it.

Here’s a picture from an agricultural area in California illustrating ~30 ft of subsidence that occurred in that area over a half century of pumping out the groundwater to water the crops.

(This also makes flooding worse… ground that used to be filled with holes that could channel the water down and away into large aquifers is now compacted such that those holes are gone. So instead water piles up on the surface in huge sheets that flow and wipe out whatever is between them and a lower elevation.)