digital camera with movie function attached to roller skate + drain
rods with a torch? stumped otherwise...

have you considered pit could just be sunk in to water table and
concrete is finally letting it through. may sound ridiculous but I have
seen a building where water table was only 3 ft below surface and
digging foundations for conservatory was enough to reveal it. in this
case you could simply "tank" (waterproof cement) the pit.

ben