Casting While Burrowed

[Caly2020]

Could a silent spell be cast while burrowing? I see the holding breath portion of the spell so I assume it would have to be cast silently.

Could I interpret the target "within view" being valid while burrowed?

[Kayerloth]

You need a valid Line of Effect and or Line of Sight to any target beyond the range of touch.

I personally would rule that a target within view doesn't literally need to be seen per se but does need to be sensed and their position pin pointed somehow, for example, Tremorsense. And LoE and LoS issues still need to be dealt with as appropriate for whatever hypothetical spell we are discussing.