Dig it up and patch it. You nearest water company has some acoustic devices that detect how far away the leak is from the probe of the device. They might help you find it.
Then you turn off the water and go to the spot and do a lot of digging until you can reveal the leak in the pipe. You can patch it in various ways, or you may have to cut it out and replace that piece of pipe.
Water companies are constantly doing this, because water seems to really want to leak.