Can you cast a spell while casting a spell?

Question

Suppose I start casting a simple spell, such as Sol Ring. I follow the steps in rule 601, up to 601.2g, which is where I get a chance to activate mana abilities. I tap Selvala, Explorer Returned, which makes me draw as part of her mana ability. My Archmage Ascension replaces the draw with a library search. While searching my library, I decide to cast Panglacial Wurm.

I imagine this means I go through the Wurm’s casting procedure, from 601.2a to 601.2i, putting it on the stack above Sol Ring, then finish applying Archmage Ascension (putting a card in my hand and shuffling), then resume the casting of Sol Ring at 601.2h (paying costs). Is this correct?

What happens if the casting of Sol Ring becomes illegal while casting the wurm? For example, I tap another Selvala for mana, replace the draw with Words of Wilding (which I had activated before starting to cast the Sol Ring), replace the token creation with Esix, Fractal Bloom, creating a copy of an opponent’s Nullhide Ferox.

Answer 1

Yes, you can do this. You laid out the steps pretty clearly, and at no time did you take an action that would be illegal. Panglacial Wurm specifically allows you to cast it at an abnormal time, while you don’t have priority.

If the spell becomes illegal, then the casting of that spell is reverse and Sol Ring would return to the zone it came from. Other mana abilities you activated could be reversed and the payments cancelled; however the activating of Selvala and the casting of Panglacial Wurm would not be reversed. You would end up with Panglacial Wurm still on the stack.

726.1. If a player takes an illegal action or starts to take an action but can’t legally complete it, the entire action is reversed and any payments already made are canceled. No abilities trigger and no effects apply as a result of an undone action. If the action was casting a spell, the spell returns to the zone it came from. Each player may also reverse any legal mana abilities that player activated while making the illegal play, unless mana from those abilities or from any triggered mana abilities they caused to trigger was spent on another mana ability that wasn’t reversed. Players may not reverse actions that moved cards to a library, moved cards from a library to any zone other than the stack, caused a library to be shuffled, or caused cards from a library to be revealed.

Note that you are not reversing the entire game back to before the illegal action (casting the spell). Rather you are only reversing the casting of the spell (and any mana payments except Selvala).

See also this ruling on Selvala:

If you activate Selvala’s ability while casting a spell, and you discover you can’t produce enough mana to pay that spell’s costs, the spell is reversed. The spell returns to whatever zone you were casting it from. You may reverse other mana abilities you activated while casting the spell, but Selvala’s ability can’t be reversed. Whatever mana that ability produced will be in your mana pool and each player will have drawn a card.

Note that you don’t need such a complicated chain of events to make Sol Ring illegal, Selvala can do that all by herself by failing to produce enough mana.

Answer 2

GendoIkari's answer covers the bulk of your question perfectly, Panglacial Wurm is one of a few cards with such weird timing requirements that it can interrupt other castings, but I want to address your last question:

What happens if the casting of Sol Ring becomes illegal while casting the wurm?

Nothing happens. Sol ring is still going to be cast. This is because the legality of the spell based on restrictions is checked in 601.2e, and you have already passed that point when this rule is checked when you activate Sevala at 601.2g, so creating a casting restriction at this point will not prevent Sol Ring from resolving. This is covered specifically in rule 601.4:

601.4. If a player is no longer allowed to cast a spell after completing its proposal (see rules 601.2a–d), the casting of the spell is illegal and the game returns to the moment before the casting of that spell was proposed (see rule 726, “Handling Illegal Actions”). It doesn’t matter if a rule or effect would make the casting of the spell illegal while determining and paying that spell’s costs (see rules 601.2f-h) or any time after the spell has been cast.

601.4 is something of a more explicit restating of the timing involved with 601.2e. This specifically lays out that if at any point after 601.2e (601.2f-h or any time after the spell has been cast), the game has passed the point in casting where it cares about legality to cast, and the casting can proceed as normal. That said, as Gendo suggested, not being able to pay violates 601.2h and the game will cause the reversible actions involved in casting the spell to be reversed for that reason still.

Answer 3

You may cast it, even if a spell is currently resolving.

Panglacial Wurm's ability is an ability that modifies another keyword action. It modifies searching in the same way that regeneration modifies destruction.

Nothing prevents keyword actions (draw, destroy, cast, search, etc) from being taken while a spell is resolving. And there's nothing special about cast in this respect.

But the confusion is understandable. The following explains why "you may cast" is weird.

---

"You may X" is usually an instruction to do the thing there and then.

• Curiosity:

  Whenever enchanted creature deals damage to an opponent, you may draw a card.

• Angelic Renewal:

  Whenever a creature is put into your graveyard from the battlefield, you may sacrifice Angelic Renewal. If you do, return that card to the battlefield.

"You may cast ..." is usually an exception.

• Flashback Keyword Ability:

  “Flashback [cost]” means “You may cast this card from your graveyard if the resulting spell is an instant or sorcery spell by paying [cost] rather than paying its mana cost”

  This modifies what you can cast when you have priority.

• Author of Shadows:

  When Author of Shadows enters the battlefield, exile all opponents’ graveyards. Choose a nonland card exiled this way. You may cast that card for as long as it remains exiled, and you may spend mana as though it were mana of any color to cast that spell.

  Same.

• Deflecting Swat:

  If you control a commander, you may cast this spell without paying its mana cost.

  This just replaces the cost to cast.

But not always.

• Bring to Light:

  Search your library for a creature, instant, or sorcery card with mana value less than or equal to the number of colors of mana spent to cast this spell, exile that card, then shuffle. You may cast that card without paying its mana cost.

  Despite having almost exactly the same wording as Deflecting Swat, this is an optional instruction to cast during resolution.

---

So which one is it for Panglacial Wurm?

While you’re searching your library, you may cast Panglacial Wurm from your library.

When it's not an instruction, "you may cast" usually modifies what you can cast when you have priority, or it modifies the cost to cast a spell. This isn't any of those things.

Instead, it modifies the "search" keyword action to add the option to cast a spell as part of that action. It doesn't matter what causes you to search, and it doesn't matter if a spell is resolving.