Over long periods of time, it is natural for sea levels to slowly rise or fall.
Sea-level rise around America’s northeast coast
Horton and fellow researchers have used that fossilized evidence to reconstruct sea-level rise around New Jersey going back 10,000 years in research newly published in the Journal of Quaternary Science. To do this, they collected sediment cores drilled tens of meters below ground from coastal marshes, then examined the sediment back in a lab for microscopic organisms that only exist at specific depths below sea level. Salt marsh grasses also fossilized within the sediment were used to radiocarbon-date the samples.
Introduction excerpted from Wikipedia:
Doggerland was an area of land that connected Great Britain to continental Europe. It is now submerged beneath the southern North Sea. It was flooded by rising sea levels around 6,500–6,200 BCE. Geological surveys have suggested that it stretched from Britain’s east coast to the Netherlands and the western coasts of Germany and the peninsula of Jutland.
It was probably a rich habitat with human habitation in the Mesolithic period, although rising sea levels gradually reduced it to low-lying islands before its final submergence, possibly following a tsunami caused by the Storegga Slide.
The archaeological potential of the area was first identified in the early 20th century, and interest intensified in 1931 when a fishing trawler operating east of the Wash dragged down up a barbed antler point that was subsequently dated to a time when the area was tundra. Vessels have dragged up remains of mammoths, lions and other animals, as well as a few prehistoric tools and weapons.[
Doggerland was named in the 1990s, after the Dogger Bank, which in turn was named after the 17th century Dutch fishing boats called doggers.
The following map is from National Geographic: (www.nationalgeographic.org/maps/doggerland)
Map by William E. McNulty and Jerome N. Cookson, National Geographic Magazine
Protecting coastal cities from rising sea levels