I worked on storm water drainage in the 1980's, back when chips were always deep fried in lard which was often discarded into the storm system rather than risk blocking a domestic outlet. So I've seen and removed a number of these "bergs" or logs as we called them. Unpleasant but at least relatively benign.
The worst situation encountered though was produced by a supermarket saving money by pouring out of date dairy products into an unsurprisingly blocked car park drainage system. Judging by the volume and eye watering smell given off by the remains it must have been going on for at least five years.
To remove the blockage required pumping out the liquor formed by peutrifaction but in order to get to that we had to break through the hard crust of rotting fat in each manhole chamber. Sewer-men hardened by years of shite-wading emptying themselves as each crust was broken was something I hope never to experience again.