10 Modern Cities That Were Underwater 100 Million Years Ago
April 25, 2026
A hundred million years ago, Earth was a very different world. Global sea levels were dramatically higher, and shallow seas covered vast areas of every continent. Many of the world's great cities sit on ground that was once beneath the waves.
Why Sea Levels Were So High
During the Cretaceous period, roughly 66 to 145 million years ago, global temperatures were 6 to 12 degrees Celsius warmer than today. There were no polar ice caps, and global sea levels stood roughly 200 to 250 meters higher than today, flooding vast continental shelves.
The mid-ocean ridges were also more active, pushing up from the ocean floor and displacing water onto the continents. This combination created the Cretaceous seaway period, when continental flooding reached its peak.
The Cities That Were Underwater
- London, United Kingdom: Central London sits on chalk deposited in a warm Cretaceous sea around 80 to 100 million years ago. The white cliffs of Dover are famous remnants of this seafloor.
- Paris, France: The limestone beneath Paris was deposited in a Cretaceous sea. The Paris Basin was a marine environment for tens of millions of years.
- Denver and Kansas City, USA: The Western Interior Seaway stretched from the Gulf of Mexico to the Arctic Ocean, putting Denver and Kansas City hundreds of meters below an inland sea.
- Dallas, USA: The Texas region was fully covered by the Western Interior Seaway. Cretaceous chalk formations across Texas were laid down in this shallow marine environment.
- Cairo, Egypt: Much of North Africa was submerged during the Cretaceous under the Tethys Sea. Marine limestone underlies large parts of Egypt.
- Moscow, Russia: Parts of the Moscow region were covered by a shallow Cretaceous sea, evidenced by chalk formations found in the area.
- Beijing, China: Parts of northern China were covered by shallow Cretaceous seas. Abundant Cretaceous fossils are found in northeastern China.
- Buenos Aires, Argentina: Parts of the Pampas region were submerged during peak Cretaceous sea level.
- Sydney, Australia: Eastern Australia experienced significant marine flooding during parts of the Cretaceous.
- Nairobi, Kenya: Much of East Africa experienced marine flooding during the Cretaceous from the Tethys Sea extending southward.
What Lived in These Ancient Seas
The shallow Cretaceous seas were teeming with life. Ammonites, coiled cephalopods related to the modern nautilus, were among the most abundant creatures. Mosasaurs, marine reptiles up to 17 meters long, were the apex predators.
The chalk deposited in these seas is made primarily of microscopic calcium carbonate shells of single-celled algae called coccolithophores. A one-meter section of chalk represents roughly 10,000 to 30,000 years of slow accumulation on an ancient seafloor.
The chalk underlying London and Paris is made of billions of microscopic shells that settled on a Cretaceous seafloor over millions of years.
Frequently Asked Questions
How high were sea levels 100 million years ago?
Global sea levels during the mid-Cretaceous were roughly 200 to 250 meters higher than today, enough to flood large portions of every continent.
Were dinosaurs alive when these cities were underwater?
Yes. The Cretaceous period was also the time of greatest dinosaur diversity. Dinosaurs roamed the land surrounding these ancient seas.
Could sea levels rise that high again?
Not anytime soon. Cretaceous sea levels required no polar ice plus more active tectonics. Current climate projections estimate under 1 meter of rise by 2100.