A transatlantic tunnel is a theoretical tunnel that would span the Atlantic Ocean between North America and Europe, and would carry mass transit of some type-trains are envisioned in most proposals. Using advanced technologies, speeds of 500 to 8,000 kilometres per hour (310 to 5,000 mph) are envisaged.[PARAGRAPH:2] Plans for such a tunnel have not progressed beyond the conceptual stage, and no one is actively pursuing such a project. Most conceptions of the tunnel envision it between the United States and the United Kingdom, or, more specifically, between New York City and London. The main barriers to constructing such a tunnel are cost-from $175 billion[2] to $12 trillion-and the limits of current materials science..[PARAGRAPH:2]Existing major tunnels, such as the Channel Tunnel and Seikan Tunnel, despite using less expensive technology than any yet proposed for the transatlantic tunnel, struggle financially..[PARAGRAPH:2]A transatlantic tunnel would be 88 times longer than the Gotthard Base Tunnel and 36 times longer than the Delaware Aqueduct. In 2003, the Discovery Channel's show Extreme Engineering aired a program entitled "Transatlantic Tunnel", which discussed the proposed tunnel concept in detail.