chance upon (verb)

1. While exploring the forest, the protagonist chanced upon a hidden cave containing a magical artifact.

2. As she strolled through the park, she chance upon her long-lost childhood friend by pure coincidence.

3. During an archaeological excavation, the team chanced upon an ancient burial site dating back to the Roman Empire.

4. In the distant future, a group of astronauts chance upon a habitable planet in a distant galaxy during their mission.