Masuria and the Masurian Lake District are known in Polish as Kraina Tysiąca Jezior and in German as Land der Tausend Seen, meaning "land of a thousand lakes." These lakes were ground out of the land by glaciers during the Pleistocene ice age around 14,000 - 15,000 years ago, when ice covered northeastern Europe.
From that period originates the horn of a reindeer found in the vicinity of Giżycko.96 By 10,000 BC this ice started to melt.Great geological changes took place and even in the last 500 years the maps showing the lagoons and peninsulas on the Baltic Sea have greatly altered in appearance.
Poland's territory extends across several geographical regions, between latitudes 49° and 55° N, and longitudes 14° and 25° E.
The largely straight coastline is indented by the Szczecin Lagoon, the Bay of Puck, and the Vistula Lagoon.The centre and parts of the north lie within the North European Plain. Rising above these lowlands is a geographical region comprising the four hilly districts of moraines and moraine-dammed lakes formed during and after the Pleistocene ice age. These lake districts are the Pomeranian Lake District, the Greater Polish Lake District, the Kashubian Lake District, and the Masurian Lake District. The Masurian Lake District is the largest of the four and covers much of north-eastern Poland.
The lake districts form part of the Baltic Ridge, a series of moraine belts along the southern shore of the Baltic Sea.Źródło: https://en.wikipedia.org/wiki/Poland#Geography
Take a trip into the higher parts of the mountains in particular when the weather conditions are conducive ago.