That's an incredible bit of engineering! Cracking open an interceptor sewer and diverting it. Interceptors tend to carry a large flow and are extremely dangerous to be in.
The surface area of the legs are small and they are made for this reason. To allow secure trenching through currenty water. All animals with legs have this.
It's just rocks and gravel that have been in there for a long time, it gets darker the longer it's been in there :) New rock and gravel have normal color.
