There is about 9 million tons of gold dissolved in the ocean, which is 180 times the amount that has been mined in human history. The gold in the oceans is very dilute, and a process for recovering it has not been found yet.
Damascus Syria is the oldest continually inhabited city, and has been inhabited for about 2000 years.
The ancient Spartans used a secret code machine to send messages from one commander to another. Each officer had a carefully made staff that was a precise diameter. A strip of cloth was wrapped around the staff, and a message was written on it. When the strip was unwound, it was unreadable without first being wrapped around a staff of the same diameter.
A flush toilet, drain pipes, and supplies lines of water were found in the palace of Knossos, dating from about 2000 BCE.
The ancient Mayans calculated the path of Venus to an accuracy of within fourteen seconds per year.
The Great Wall of China was not built to keep nomadic invaders from entering the Chinese homeland. They were built to keep out the horses of the invaders. Without their horses, the nomadic tribes were not that formidible. The Wall also slowed down the invading army, allowing nearby garrisons to respond before the whole army could penetrate the Wall.