One time I was exploring without any blocks or tools and fell down a hole. Had to either starve or use the /suicide command.
It would be awesome if we could climb up flat, verticle surfaces. However, it wouldn't be that simple- there'd be a grip meter.
To start climbing vertically, go up to the verticle surface in question and press a specific key. That will have you cling to the wall. Then hold W to ascend, and the ASD keys to move in their specific directions.
Once you cling to a wall, in the top right it'll show your current grip strength- it starts at 100%.
Everytime you move against a block while gripped to it, be it verticle or horizontally, that percentage goes down based on the kind of block it is. Slicker blocks would lower the grip percentage much quicker.
If it's raining on the blocks you're trying to grip against, grip percentange goes down twice as fast.
If you press the jump button you'll leap in the direction specific to your WASD keys. However, once that leap finishes, you'll lose a lot of grip.
Once grip percentage reaches 0%, you slip and fall, and have to wait until it fills back up to 100%.
Wearing gloves can lower the rate at which grip is lost. Getting damaged while climbing will lower grip by a lot. Climbing while hunger is below 50% will make grip drain faster.