I give the player a bed to start with (except in hard mode) as resources can be really hard to come by above the world and getting below will take sometime
the shell over the world (hollow earth below) is grass, some dirt, some stone, and an obsidian layer. getting down. isn't that hard but it can take time... the shell extends as you move (above or below the top layer) it's edge is 80 blocks away
if you do get bellow the shell it's going to be dark all the time.... probably. (minecraft can be buggy... or maybe you'll carve a giant hole)
the shell does have saplings on it from time to time. life above is possible. Also, stuff can poke through... trees and some mountains. the higher the shell the more rare these are. this makes for an easy way down if you can find something poking through.
on rare occasion (more common on peaceful,easy...slightly rare on normal and rather rare on hard) you'll find a home with a villager on the shell. He's got a few resources that can make life above on the shell much nicer.
When you start the game on hard you get nothing you also start at a height of 170 (vs 140 on normal, 120 easy, and 90 on peaceful) things poke through the shell much more rarely on hard and the fall down is farther.