How was the undersea tunnel connecting Denmark and Sweden built?

Cars and trains that were running on the sea suddenly dip down and disappear into the sea. This was because there was an airport flight path above and a shipping lane for large vessels below, making it impossible to build a bridge either high or low. So, they decided to hide it completely underwater. This undersea tunnel wasn't dug; it was assembled. They built 20 concrete boxes, each 176 meters long, on land and submerged them one by one into a trench on the seabed, fitting them together like a puzzle. They even used the excavated soil to create an artificial island in the middle, so nothing went to waste.