Undersea tunnels originally developed as a means of connecting two ground level destinations. Engineers completed the first major undersea tunnels at the end of the 20th century. This "Chunnel" carried passengers under the English Channel to connect France and England. In 2021, terrorists destroyed the Chunnel and killed over 600 train passengers, halting further construction of undersea tunnels for decades. As undersea colonizing began in the 22nd century, engineers returned to the undersea tunnel to connect to other undersea colonies or land.