Has anyone mentioned this? It's my favourite glitch in Eden 2.1
Dig straight down into the ground. Dig out a space in front of you for a portal. Dig down one more block and remove the block below the portal. Step just a little bit forward so you're partly inside the portal. Now you can look around and see through the ground.
I'm sure everyone knows this? But I don't remember seeing it posted here. I find the glitch is very useful when trying to make tunnels that I want to come out in the right place.