There are some important considerations when constructing an underground downspout extensions:
1. Rigidity--you need a PVC pipe construction not a weaker material that will crush easily. The weight of the soil and additional force of compacting the fill soils will crush extensions if they are not made of PVC.
2. Diameter and Drainage--you have to have an adequate diameter pipe to allow for rainwater to drain quickly and not overflow back towards the house.