What is this?

http://i728.photobucket.com/albums/ww283/xuerebx/whatisthis.jpg

Is it bedrock or some kind of mineral? I tried breaking it with an iron pickaxe but nothing happened (I kept hitting it for over a minute).

it is indestructible.

Those blocks are near bottom of the world, so watch your step there.

Thanks. First time I hit so far down.

...and speaking of the bottom of the world... Post limit.