The bedrock layer is present in both the Overworld and Nether realm. Though players can build structures over the Nether realm's bedrock layer, they cannot do so underneath one in both realms.

Bedrock is the layer at the bottom of what geologists refer to as the "critical zone" because its cracks and fractures provide pathways for air and water, which break down rock and form the soil ...

image: Weathered rock (light brown), an often thick layer of fractured rock underlying the thin veneer of soil (dark brown) and just above the impermeable bedrock (gray), can hold more water than ...

The thin layer of bedrock below the Earth's surface is the foundation for all life on land. Cracks and fractures within bedrock provide pathways for air and water, which chemically react to break ...

So, that new ice is expanding, putting extra pressure on the soil and the bedrock layer underneath. Eventually, that pressure continues to build up until the soil and the rock cracks underneath.

How to build underneath bedrock layers in Minecraft 1.21 The right placement of blocks can create a starting point for building under bedrock (Image via Decapit8/YouTube) ...