There is a rumour that Delaware is the only State without even a single cave in the USA. This is not true: Delaware has exactly one cave. However, it is a wild cave and in order to protect it we will not publish anything about it.
Delaware is the second-smallest state in the United States, located on a peninsula which heads down south, along the Mid-Atlantic Coast. It is bordered to the east by the huge estuary of the Delaware river. The whole country is level, low (less than 137m asl), part of the the Atlantic Coastal Plain with flat, sandy, and, in some parts, swampy ground. The ground is flat, sandy, and swampy. Only along the northern border is some hillcountry.
The main underground thing about Delaware is the Underground Railroad. For non American visitors: this was an underground organization which helped slaves before the civil war to escape to the northern states. There is not much physically underground about this railroad, and definitely there is no railroad.