[The rush of the waterfall can be heard first, before Sherlock speaks:]

The Undercroft is holding. [A crash, a distant roar.] For the moment... [A short, tense beat as he pauses, listening.] Bursts of flame... fire magic. If it breaks through the foundations, the combustion grenades could bring the whole castle down.

[Did he say that out loud? Well. Maybe it's for the better people know what's at stake.]

Get it out of here!