Ummmm ....What?

[ScobraCK]

First, I'm new so tell me if this is the wrong thread.
Well I'm playing modded terraria(Thorium, Tremor) on PC expert mode. And I just recently defeated skeletron and found this.

Can somebody tell me how this is possible? Or if this is a bug?

 

[Rossmallo]

Sadly this can happen on very rare occasions. Go around it, or get to the Underworld and get a Pwnhammer.

 

[Games ✔︎]

Thats a huge bummer. Unfortunately this happens to others as well. As @Rossmallo said, your best bet is to obtain a Deathbringer Pickaxe as this can mine dungeon bricks.