The Baltic Sea is considered the youngest sea on Earth, developing in its modern post-Ice Age history roughly 10,000 years ago. Its formation followed the retreat of the massive glaciers that covered northern Europe.
The Baltic basin has passed through several distinct stages. As the ice sheet retreated, meltwater first formed the Baltic Ice Lake. Later, connections to the ocean opened and closed as land rebounded upward after the weight of the ice disappeared. These changes produced stages known as the Yoldia Sea, Ancylus Lake, Mastogloia Sea, and Littorina Sea.
The modern Baltic is a shallow, nearly enclosed arm of the Atlantic connected to the North Sea through the Danish Straits. Because relatively little saltwater enters but many rivers supply freshwater, it is one of the world’s largest brackish-water bodies. Salinity is generally higher near the entrances and lower in the northern and eastern areas.
The phrase “youngest sea” refers to the age of the present basin and its postglacial development, not to the age of the underlying rocks. A related fact is that the Baltic coastline is still changing: in parts of Finland and Sweden, land continues to rise through postglacial rebound.