The only problem I have is that the infiltration trench is already in the ground and a hole has formed... At one spot the ground has collapsed and a 1.8m deep hole with almost 40cm diameter has appeared. I held a camera on a long rod down there and a small cavity formed at the bottom. That was in the bed where there was only soil and where the collapse happened. I have a cistern connected upstream... I will first make sure that the cistern does not overflow and thus the infiltration trench is practically not needed. Question 1: Can I find out if there are other holes/cavities without damaging my entire lawn? Question 2: If the hole or holes are refilled, who tells me they won’t form again?