High density, robotic warehouse systems and processes may include one or more blocks of a storage system, each block including a plurality of floors, and each floor including a plurality of storage grid locations for respective totes. A plurality of first robotic drive units may transfer barges carrying totes between various processing stations and the storage system. A plurality of second robotic drive units may operate within the storage system and traverse the storage grid locations using a plurality of highway grids and elevators to move totes between barges and storage grid locations. The freely movable robotic drive units, barges, and totes enable high flexibility, modularity, scalability, and serviceability of the high density, robotic warehouse systems.