I'm a player in a campaign that takes place in the Thay / Mulhorand region. Right now we are on Alaor Island, specifically the Styes. My PC is doing some investigating and part of that involves a potential connection to the Underdark.

My question: to anyone's knowledge of DnD geography, does the Underdark underground network of caves and tunnels reach anywhere near the coasts surrounding Alaor Island / Styes, or even underwater and straight into the island?

That’s mostly up to your DM to tell you. There’s probably entrances to the under dark everywhere, but the DM ultimately decides where they are and how accessible they are.

Ya know, after reading some of the descriptions of this place, that was the impression I got, too. That's it's supposed to be open ended enough for the DM to make something of it. Thanks

i can't recall ever reading about it in a novel, but there's been tunnels connecting the Underdark to Evermeet, way out in the middle of the ocean (when it was invaded by the drow and that giant elf-eater worm thing)....so since that's possible, I'd say there's certainly tunnels to Thay, even if they took some custom digging to connect pocks of caves.