I hate to pick on Mkt (Rob), but he was TOTALLY wrong about this.
The lagoon is GONE now, and it was done very slowly, the drainage took from April 19 until almost mid June. It could've been done MUCH more quickly. They drained it slowly...and nope it didn't crumble.
I really don't see the purpose in draining a perfectly pretty (though idle) lagoon in favor of a garden. The Lagoon was (although I haven't, obviously, SEEN the "upcoming" garden) PROBABLY much, much prettier. The waterfalls from the Cave of 20K (turned off a few years ago) and Ariel's Grotto created a nice feeling.
Now it'll be some flowers.