It is my general perception that the Waterdeep plateau is holed as a swiss cheese with dungeons and the only thing keeping it structurally safe is the mythal.
Without it, prepare for landslides, earthquakes, sinkholes, realestate sliding into the seas and general mayhem. Indeed! There are massive physical dwarf-built pillars and braces, and powerful elven magics that do the most to hold the whole area up from sinkhole collapse and inundation by the sea, but the mythal is both anchored in the elven magics, and bolsters them.#Realmslore
— Ed Greenwood (@TheEdVerse) December 10, 2021
2)
There are so many layers of magic, and so many interwoven Weave-anchors, that no one living remembers them all (Weavemasters can see them), and no one can cast enough spells, quickly enough and in the right locations, to collapse the wards and mythal and…3)
…supporting magics. The last to try were the Twisted Rune, and they were then surprised to discover their efforts alerted Azuth and Mystra and several of her servitors, all more magically powerful (and loaded with battle-magics) than the Rune members.#Realmslore— Ed Greenwood (@TheEdVerse) December 10, 2021
4)
Hence the caution and operating behind the scenes that most of the Twisted Rune members have practised ever since. ;}#Realmslore— Ed Greenwood (@TheEdVerse) December 10, 2021