Albert Barnes Commentary Jeremiah 31:40

"And the whole valley of the dead bodies and of the ashes, and all the fields unto the brook Kidron, unto the corner of the horse gate toward the east, shall be holy unto Jehovah; it shall not be plucked up, nor thrown down any more for ever." — Jeremiah 31:40 (ASV)

The whole valley of the dead bodies - Probably some part of the Valley of Ben-Hinnom. Comparing Zechariah 2:4, the conclusion seems evident that Jeremiah’s words are to be spiritually understood. His city is one that renders holy to Yahweh what was before unclean. Compare John’s new Jerusalem (Revelation 21:27).