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 unto Yahweh what was before unclean. Compare John’s new Jerusalem Revelation 21:27.
Related Commentaries of Jeremiah 31:40
Jeremiah 31:40
40 And the whole valley of the dead bodies, and of the ashes, and all the fields unto the brook of Kidron, unto the corner of the horse gate toward the east, shall be holy unto the LORD; it shall not be plucked up, nor thrown down any more for ever.