Complex sentence: It was created as an'accidental' byproduct of urbanization in that area, especially mining, but very important was the specific geological structure-average thickness of sand layer is about 40 meters (maximum 70 meters) which made fast and deep drainage very easy.
Split into simpler sentences: It was created as an'accidental' byproduct of urbanization in that area, especially mining. The specific geological structure has been of big importance-the average thickness of the sand layer is about 40 meters (maximum 70 meters), which made the fast and deep drainage very easy.