A system for transporting inventory items includes an inventory holder and a mobile drive unit. The inventory holder has a frame with device openings. The mobile drive unit is operable to dock with an inventory holder and undock with an inventory holder. When undocked from an inventory holder, the mobile drive unit is further operable to traverse a path from a first location to a second location. The path passes through at least a first device opening of the inventory holder such that the mobile drive unit passes beneath the inventory holder while moving along the path.