Why is the isle of thanet not an island?

The Isle of Thanet is actually a peninsula in south-east England, not an island. It is located in the county of Kent and juts out into the North Sea. While it is surrounded by water on three sides, it is still connected to the mainland by a narrow strip of land. Historically, the Isle of Thanet was an island, but over time, the sea levels dropped and the land bridge formed, connecting it to the mainland.