Common construction methods of root cellars: Digging down into the ground and erecting a shed or house over the cellar. Digging into the side of a hill (easier to excavate and facilitates water drainage). Building a structure at ground level and piling rocks, earth, and/or sod around and over it. This may be easier to build on rocky terrain where excavation is difficult. Most root cellars were built using stone, wood, cement, and sod. Newer ones may be made of concrete with sod on top.
illustration- Alternatives to downspout extensions-explains a hard way but then you could buy coil up thing to attach that uncoils when water comes out gutter & then coils back up- He says he han't tried it though- but i think it's worth a try