A passage in the Cavern.
King Jaron had plans to widen the Passage to allow for further expansion in the Cavern. In 6430 he made the announcement. Rock Biter and The Burrower were both christened in a great ceremony and immediately put to work.
In 6510 they discovered a new cavern, however the Guild of Surveyors (led by King Rikooth's older brother) brought detailed reports (structural analysis, water supply, rock type, etc.) indicated that it would be very expensive to build in. After confering with his advisers, he ordered the Burrower and Stone Eater to continue to tunnel past the cavern.