Where does all the extra earth come from?

When watching Time Team, everyone's favourite Sunday evening program, they find some incredible things with their archeological digs.

Walls, building remains, treasure, evidence of past settlements etc etc.

But why is it all under the surface? Where does the large amount of earth come from that covers it? Is the landscape constantly changing?

Some digs go down several meters and find an ornate tiled floor - that's a lot of earth now covering it!

It's always bugged me. Thanks for any insight!

EDIT: Many thanks for your replies. I guess I underestimate the power of nature and the ever changing world. Learning from Reddit is cool learning.