This conference talk explores Twitter's journey of migrating "Source," their massive monorepo containing nearly 20 million lines of hand-crafted code and 200 million lines of generated code, from the Pants build tool to Bazel. Discover how Twitter approached this significant migration for their predominantly Scala codebase, which also includes Java, Python, NodeJS, Go, and C/C++ components. Learn about the current state of the migration, the unique challenges encountered when transitioning from Pants to Bazel, and the strategies employed to overcome these obstacles. Speaker Borja Lorente shares insights from this large-scale build system migration during his 16-minute presentation at BazelCon 2020.