An early version of Chrono Trigger had an area in 65,000,000 B.C. with outdoor waterfalls and indoor lavafalls. Singing Mountain was not in the final game, but it looks fairly complete. The official Chrono Trigger soundtrack even had a unique song for the level in addition to an unused battle theme. Since the assets are already done Square Enix could use this as one of the new dungeons (note: plural) in Chrono Trigger DS.
Do you think we’re going to explore Singing Mountain in November?
As a side note Square Enix posted the official box art and it doesn't use Echoes of Time as a subtitle. Square Enix spent money to trademark Echoes of Time so this could be a sign that the DS remake is a Valkyrie Profile Lenneth-like spark for the Chrono series.
Images courtesy of Chrono Compendium and Square Enix.