The path is a long, dark hallway that travels under Saffron City. Despite the cave-like lighting of the path, there is no threat of being attacked by wild Pokémon. Occasionally, by using an Itemfinder, items that Trainers have dropped along the path can be found and retrieved.
The only difference across all four generations was the removal of the Underground Path as a playable area in Generations II and IV. The reason is explained in a note from the Celadon Police left on the signpost to the Underground Path in Route 7.