Forget your diving rods, joss sticks and general poking around when prospecting for new cave. This is what you need -
http://www.telegraph.co.uk/news/uknews/defence/12201086/MoD-gravity-sensor-breakthrough-to-see-underground-or-through-walls.html
http://www.telegraph.co.uk/news/uknews/defence/12201086/MoD-gravity-sensor-breakthrough-to-see-underground-or-through-walls.html