We have 2 septic systems. The house has 2 tanks, a leach field, and was 36" deep to the lids. When it came time to pump them the probe to find the lids went to the handle at the ground. The system for the stables has 3 tanks, a single 240' drain line, and they were only 24" underground, so easier to find. Now all 5 tanks have risers putting the lids 12" underground, much easier to locate. I try not to run over them with anything heavy. They get pumped every 3 years. We are in sand so they drain quite well, easy to spot the green grass where the leach field is.