The Starlink Hierarchical Data System has been a very successful niche astronomy file format and library for over 30 years. Development of the library was frozen ten years ago when funding for Starlink was stopped and almost no-one remains who understands the implementation details. To ensure the long-term sustainability of the Starlink application software and to make the extensible N-Dimensional Data Format accessible to a broader range of users, we propose to re-implement the HDS library application interface as a layer on top of the Hierarchical Data Format version 5. We present an overview of the new implementation of version 5 of the HDS file format and describe differences between the expectations of the HDS and HDF5 library interfaces. We finish by comparing the old and new HDS implementations by looking at a comparison of file sizes and by comparing performance benchmarks.