that's a small pipe for a pool. is it white pvc? is it from a heater? do you have a polaris pool cleaner? how far below the skimmer and drain is the end of the pipe in the street? could there be a leak under the pool and this is a drain line to prevent cave-ins? are your refilling the pool while it's leaking? What happens if you plug the drain line? Is there back up any where?

Just wondering?