Learn Spanish (Latin America): Ustedes saben que la corteza se viene rompiendo. - You all know that the crust has been breaking.

Using venir plus a gerund (like rompiendo) is a great way to show that an action started in the past and has been gradually progressing or building up to the present. While ha estado rompiendo also means "has been breaking," viene rompiendo emphasizes the ongoing, accumulating process of the crust falling apart.

Why is rompiendo reflexive here (using se)?

The verb romper means "to break [something]." When an object breaks or undergoes a change of state without a specific person causing it, Spanish uses the reflexive form romperse. Since the crust is breaking by itself, we need that se.

Where else could the pronoun se go in this sentence?

When you have a conjugated verb followed by a gerund (viene rompiendo), you can put the pronoun before the conjugated verb (se viene rompiendo) or attach it directly to the end of the gerund (viene rompiéndose). If you attach it, you have to add an accent mark to keep the stress on the right syllable!

Is Ustedes only used in formal situations?