So the next challenge I thought up was a 'one tree challenge': Spawn, find the closest tree and chop it down, then dig directly down where the tree stood and never come back up again. Since wood is needed for a workbench, the first pick, handles and torches, you really need to think about what you dig. Hardcore mode as well, death = regenerate the world.
The first attempt I dug around the shaft of light to the bottom of the map, but didn't hit any coal to get torches going. Tried getting a space big enough for a tree to grow at the bottom, but eventually ran out of handles.
2nd attempt I tried a shallow tree directly under the shaft of light to see if it was even possible to grow them underground, but I punched through the roof of an enormous cavern just below my starting box. There was a waterfall and pool right next to where the shaft of light landed, so I plonked a tree there and waited.
There was also some coal available, so risking some wood for a set of torches helped me explore the caverns a little as well as making night-time mining a little more fun. I'd also started spreading grass down from the top by rebuilding the staircase with dirt. Don't know if it's required for trees to grow, but it can't hurt (and digging out dirt saves the handles while I wait for the tree).
A couple of days had passed and I was getting worried about the tree. If it doesn't grow then it'll be a pretty useless challenge, but if you're able to build them anywhere it'll also be somewhat useless once you get going. I had one more seedling from the original tree, so I tried another spot just in case, and splurged another couple of sticks for torches to ring the seedlings. Please work!
Coming back from another spelunking expedition (groans are pretty scary when you have no equipment and only 4 torches to cycle through) I noticed the greenery. Yes! Trees! The bottleneck was more or less gone now, but I kept up preparing the initial cavern for more trees just in case there was an accident. I also splurged on a hoe to place some wheat next to the tree trunks. Not sure whether the grass triggered the trees growing, but it was essential to get the farming up and running. 1 chook had managed to fall through the hole, but pigs couldn't fit so I needed the bread in case the groans became serious.
Greenified the 2nd cavern and established a channel for the water to follow my journey deeper into the cave system. I'd seen some lava, and had learned from previous adventures that being on fire is lethal without water handy. Eventually made some iron armour for confidence to push further down, but died to a cave-in before it was put to the test. I'd followed a cave heading up toward the top and had simply placed a torch near some sand when it all came down on me.
With trees able to be grown almost anywhere once you have torches, the challenge loses its teeth after the first tree grows. The initial resource limitations certainly helped give it a more desperate feel which carried long past the point of safety, so overall I'm pretty happy how the challenge went. It's a pity that Multiplayer has infinite health and no durability issues as it would have been an interesting challenge with a couple of people. Might try it again at the LAN anyway.