Some, now exhausted, former open-pit mines are now being converted into artificial lakes, with the hope of attracting holiday-makers, and the area is now being referred to as the Lusatian Lake District ("Lausitzer Seenland"). The Lusatian Lake District (German: "Lausitzer Seenland", Lower Sorbian: "Łužyska jazorina", Upper Sorbian: "Łužiska jězorina") will become Europe's largest artificial lake district. Some of the biggest lakes are Lake Senftenberg ("Senftenberger See" / "Złokomorowski jazor") and Bluno Southern Lake ("Blunoer Südsee" / "Južny Blunjanski jězor"). The Upper Lusatian Heath and Pond Landscape (German: "Oberlausitzer Heide- und Teichlandschaft", Upper Sorbian: "Hornjołužiska hola a hatowa krajina") is the region richest in ponds in Germany, and together with the Lower Lusatian Pond Landscape forms the biggest pond landscape in Central Europe. As Lusatia is not, and never has been, a single administrative unit, Upper and Lower Lusatia have different, but in some respects similar, histories. The city of Cottbus is the largest in the region, and though it is recognized as the cultural capital of Lower Lusatia, it was a Brandenburg exclave since 1445. Historically, the administrative centres of Lower Lusatia were at Luckau and Lübben, while the historical capital of Upper Lusatia is Bautzen. Since 1945, when a small part of Lusatia east of the Oder–Neisse line was incorporated into Poland, Żary has been touted as the capital of Polish Lusatia. According to the earliest records, the area was settled by culturally Celtic tribes. Later, around 100 BC, the Germanic Semnones settled in that area. The name of the region may be derived from that of the Ligians. From around 600 onwards, West Slavic tribes known as the Milceni and Lusici settled permanently in the region. In the 10th century, the region came under the influence of the Kingdom of Germany, starting with the 928 eastern campaigns of King Henry the Fowler. Until 963 the Lusatian tribes were subdued by the Saxon margrave Gero and upon his death two years later, the March of Lusatia was established on the territory of today's Lower Lusatia and remained with the Holy Roman Empire, while the adjacent Northern March again got lost in the Slavic uprising of 983.