I have experimented with underground tree growing. I would channel a big square room out and build a wall around it on the surface (so things cant wander in on it). The ground will have to be soil for the trees to grow. Wait a few seasons and grass will start growing. Then little saplings will start popping up.
Of course, if you're going to build a wall around it, you don't need to channel out the area at all, do you?
Note, too, that it's easy to see where a hidden chasm is (though this is probably a game exploit that needs to be fixed). Or it was on my map, anyway, but this is the first chasm I've seen.
I'd post the method here if I had the slightest idea of how to do that and hide it under a 'Spoiler' tag,... but I don't. And I don't suppose it's any big secret, since it was pretty obvious. But I haven't been able to find my underground river. I've got lots of dead creatures, but can't zoom to any of them, and I couldn't even right at the start.
Oh, well, it took lots of searching to find the magma pipe, so I've got a bunch of legendary miners. I'll find the river eventually.
Bill
PS. OK, maybe I see how this is done. Here's how I found my chasm. As I say, it's probably no secret, but don't look if you don't want to know.
In the Stocks screen, cave spider webs show up under 'thread.' Even if you can't see them, and haven't found the chasm, you can still zoom to them, so that tells you where to dig.