More area underground than above ground

The attached map shows how the Domaniale Mine concession was partly located in what used to be the Kingdom of Prussia.
The Concession Border is shown in yellow on the map, and reveals that the area of the Netherlands underground was bigger than the area above ground!
The Final Act of the Congress of Vienna (1815) abruptly separated Kerkrade and ‘s-Hertogenrade; two municipalities that had formed a socio-economic unity for centuries. This created a strange situation, especially in ‘s-Hertogenrade, as one half now belonged to the Netherlands and the other half to Prussia.
This situation was resolved by the border separation treaty signed by the King of the Netherlands and the King of Prussia (Aachen Borders Treaty of 26 June 1816).
Part of the area along the Wurm was ceded to Prussia, including the centres of Kohlberg, Maubach, Pesch, and Strass. The road from Aachen to Geilenkirchen (the current Nieuwstraat) and the entire area east of it became part of Prussia.
The current Nieuwstraat was an important trade route for Prussia.
At that time, the Municipality of ‘s-Hertogenrade was renamed Herzogenrath.
The Aachen borders treaty resulted in part of the concession area (173 hectares) becoming part of Prussian territory. This did not affect the Domaniale mines, because Article 19 stipulated that the Prussian government could under no circumstances interfere with the exploitation, impose restrictions on extraction, or levy taxes or tolls on the extraction and export of coal.

(Source: ‘Domaniale Steenkolenmijnen 1815-1996’ [Domaniale Coal Mines 1815-1996] by Paul Geilenkirchen)