Where to find Shadow Orbs and how to get at them?

[ApathicAlpaca]

So I did some bosses pre-hardmode but I can't summon the corruption worm. I read you need to destroy Shadow Orbs. OK. I checked every single corruption 'gulch' accessible from the surface and there was not a single orb.
The corruption stones are indestructible so there's no way to get into the subsurface corruption areas... or do I need to use massive amounts of bombs or stuff to blow walls up?
Anyway, I only ever saw altars but never Shadow Orbs. Where do I find them?

Thanks.

[prpl_mage]

The Orbs are inside the chasms, they have a purple shine to them. When you jump down into the bottom of the chasms in the corruption you usually find them.

And you are correct, you need bombs or purification powder to get through"!

[ApathicAlpaca]

With chasm you mean the ones you can just jump into from ground level (surface)?
Because I found like 12 of them on the map and none of them had a an orb inside, not at the bottom or anywhere else. They do have a small destructible thingy that drops some coins but that's it.