Auvergne has about 50 freshwater ponds and lakes. Some are high in the mountains and have volcanic origins. Lac de Guéry is the highest lake in Auvergne. Auvergne is bordered to the north by the region of Centre-Val de Loire, and by five former administrative regions: Rhône-Alpes to the east, Languedoc-Roussillon and Midi-Pyrénées to the south, Limousin to the west, and Burgundy to the north. The average annual temperature is , and the region receives 510–1,020 mm of rainfall annually. There are long winters and short summers. The region of Auvergne was named after the Arverni, one of the most powerful Gallic tribes. It was composed of the Gabali, the Vellavi, and the Cadurci, whose sphere of influence included the regions of Languedoc and Aquitaine. Vercingetorix was elected king in 52 BC. His father, Celtillos, his predecessor, had been killed by his companions who opposed Celtillos' goal of making the title hereditary. In the winter of 53/52 BC, Vercingetorix created alliances with all the Celtic tribes surrounding him by holding as hostages daughters or sons of the kings of each tribe. With this threat, he gained their guarantees of faithfulness and alliance. Based on reports in 2007 of excavations by archaeologists (radio programme of Yves Calvi with researchers in October 2007), the capital of the Arverni is believed to have been situated between Gergovie, Corent, Aulnat and several other significant areas within a 35 km range. Researchers estimate a population of 150,000 inhabitants living in the centre of this area, and a total of more than 400,000 inhabitants living in the region of these towns. The Arverni were one of the most powerful and wealthy tribes in ancient Gaul:  A shrine in Auvergne marks the Battle of Gergovia. Based on scholars' interpretation of books by Caesar, it took place about 12 km from present-day Clermont-Ferrand; this has not been conclusively proved. Vercingetorix beat Julius Caesar at Gergovia in 52 BC before he started chasing Caesar’s troops. Roman troops won a victory in Alesia (Alise-sainte-Reine) in Burgundy. Roman legionaries had set traps and established over several hundred metres.