Via the forums of FrankC's Lost Levels, there's a some discussion on an unreleased N64 game prototype currently up for eBay auction, named 'Echo Delta'.

The discussion points out that there was an IGN preview of the game back in 2000, explaining of it: "Echo-Delta, developed by the Marigul-funded design team Clever Trick, is a title that's hard to categorize. It has both real-time strategy (RTS) and RPG elements, but plays like an action-based game as well. You have 18 minutes to raise a sunken ship from the ocean floor. To do this you scour the sea bottom in search of resources that can be used to help upgrade your special submarine named Scout." Interesting stuff.

Looks like there are other copies of this undumped proto floating around, though - there's a long and fascinating conversation about another (?) copy on the ASSEMbler Forums back in 2004, which also goes into whether there are a number of other Spaceworld 2000 N64 protos floating around - apparently so! The current seller wants $1,520 for this Echo Delta N64 proto cart, though - pretty pricey.