These services cannot be directly accessed but forms the underlying infrastructure for many of Google Cloud Services
Spanner
Globally-consistent, scalable relational database
Cloud Spanner is the external offering of this service
Borg
A cluster manager that runs hundreds of jobs, from many different applications across a number of clusters each with thousands of machines
The open-sourced version of Borg became Kubernetes
Chubby
A distributed lock manager (DLM) as a service that temporarily prevents files and records from being used by another user or operation on a Virtual Machine
Colossus
Cluster-level file system, successor to the Google File System (GFS) provides the underlying infrastructure for all GCP Storage Services, from Firebase to Cloud SQL to Cloud Storage