Explode Block Definitions
The block reference can be exploded into its component objects, if you need to modify one or more objects within the block separately.
By selecting the Explode option in the INSERT dialog box, you can automatically explode block references as you insert them.
When you explode a block with nested block, only the top-level block will be exploded, the nested block not be exploded.
Even if all references of a block are exploder or deleted, the block definitions is still stored in the drawing and can be inserted.