Historically cities were hit by floods and wars and new buildings were built on top of the foundations of old ones. We had an article about that church in Rome built over another roman church built over another roman church, etc. down to an old temple on a spring, or something like that.

Buried ships of San Francisco - https://www.nps.gov/safr/learn/historyculture/buried-ships-o...

https://www.baylightscharters.com/bay-lights-charters-blog/w...

> Delgado received his first big assignment back in 1978 while working for the National Park Service: excavating and studying the remains of the Niantic, one of the first whaling vessels that brought gold-seekers to the area. It had been discovered near the Transamerica Pyramid at the corner of Clay and Sansome streets. After being left behind during the Gold Rush, the ship had been repurposed to serve as a storeship, saloon, and hotel until its demise in an 1851 fire.

Consider that https://maps.app.goo.gl/tYjaESQXss2KhHXQA used to be sea level.

As mentioned else comment, things were torn down and that served as the foundation for the next building.

Walking around Chicago I often see houses where the front door is a couple of meters below street level because the house never moved its door to an upper story when the city was releveled.