Um... is this game supposed to have bottomless pits? With trees growing over emptiness? and rivers disappearing abruptly? I'm afraid to go in because I can't see the bottom.
In case this is a bug someone wants to look at, I did a save-as of the server at this point. Then I placed a single block on the side of a block inside it and land was suddenly generated around it up to a point. There was a smaller ledge that I could jump into, which was filled in when I placed the single block, so that's scary, but when I cut a tree nearby to drop in the rest of the chasm filled and the river connected.
What's really scary about this is if a player can fall in and tries to get out by filling in blocks. It would be so much worse than getting stuck between blocks while building a house, and the game doesn't seem to let you dig while stuck, so it would literally be a game ending bug for a player unless some button to "unstuck" a player by shifting them to the nearest unoccupied space is possible.