The decreasing altitude of western Netherlands

Most of western Netherlands is below sea-level; but this is not land claimed from the sea.

This was bogland, organic delta soil just above sea-level. It was partially habitable, even without serious hydro-engineering.

But the engineering that did occur — drainage of wetland — started a long, slow decrease in elevation.

The primary effect was the simple loss of water. Water is a large component of peat, so drainage removed soil volume immediately.

The next effect takes longer but is more profound over time. When peat is exposed to air it reacts with oxygen, and decomposes. The organic soil breaks down into water, CO2, and simple minerals. It all disappears, and this loss of mass continuously, slowly decreases the volume of the soil.