A method for treating large bodies of water contaminated with heavy metals and having varying density stratas to immobilize the contaminant metals is disclosed. When a length change is to be made, hydraulic fluid unlocks a locked one of the locking mechanisms, allowing inertial force to effect the length change during an engine cycle. By forming a void in the lower trench, a dielectric constant at the bottom of the isolation layer is lower than a dielectric constant at an oxide layer, thereby improving the isolation property.