Initial observations suggested that the issue was a drainage fault. Upon further investigation, it was found that there was a large hole in the wall of the inspection chamber and clear evidence of rat activity.
Rats use drainage systems because they provide a safe and easy way for them to travel and find food and shelter. Drainage systems, such as sewers offer protection from predators and the elements, and they also often lead to areas where there is a concentration of food sources, such as rubbish or compost.
Additionally, rats are good swimmers and can easily navigate through water, making drainage systems an attractive option for them to move around in.