"That would be an advantage for something like this," Hatalath says. "Channelling it through the air would be easier, but it would also be disrupted the first time a bird flew through it, let alone what a storm of magic would do. Planting the leylines in stone frontloads your difficulties - it's a huge challenge to set up, but once it's set up it'll last."
"Magic may not flow through stone particularly quickly, but it does so reliably," Thorek says in agreement. "It doesn't change its speed based on the time of day or phase of the moon or because of the emotions of passers-by."
"I'll grant you that, but that answers the 'why', not the 'how'. If you just pumped magic deep underground it'd radiate out in every direction, it wouldn't conveniently flow to wherever you directed it."
"Momentum?" Elrisse hazards. "A constantly-maintained effect that makes the leylines the most conducive path?"
"Or a permanent change," Sarvoi says, "either comparable to an enchantment, or an actual physical channel running between the Waystones."
"Whatever it is, either it's inherently reversible or bidirectional, or the Ice Witches are already capable of recreating them, considering that magic currently flows from Kislev City to Praag."