Geography

'Basic' D&D dropped players into a town by the name of threshold in the hilly north of The Grand Duchy of Karameikos, a autonomous region of The Empire of Thyatis in The Known World, the south east corner of Brun.

The Empire of Thyatis controls regions to the south roughly the size of the entirety of the known world and is locked in a cold war with the The Alphatian Empire which formerly controlled the areas now controlled by Thyatis is around twice its side to its west. The current point of contension between the two sides in The Kingdom of Nordwold.

Nordwold is a vast tract of land with untapped mineral wealth which would give Alphatia a foothold on Brun which they have lacked since the rebellion of Thyatis while it would give Thyatis the ability to begin to encircle Alphatia. In theory Nordwold owns allegiance to Alphatia but in practice it is virtually lawless.

The Known World itself is a patchwork of human countries (with the exception of Rockhome and Alfheim) which tend to be supporters of Thyatis or at the very least opponents of Alphatia.

To the east is the great wastes, a land of bandits and humanoids mysteriously united and threatening to destroy the known world as we know it.

Maps
The map in the background is cut together from the fantasy empires game.

I would suggest the Mystara Map Navigator. For all of Darkblood's comments about the improvements in web technology, 70 MB still loads at a snail's pace and is ungainly to navigate (took me 45 seconds).

If you would like to ignore my warnings then the map I speak of is available here.

The Known World AC1000

 * Kingdom of Alfheim
 * The Atruaghin Clans
 * The Broken Lands
 * The Republic of Darokin
 * The Ethengar Khanates
 * Five Shires
 * The Principalities of Glantri
 * Heldann
 * The Kingdom of Ierendi
 * Grand Duchy of Karameikos
 * Minrothad Guilds
 * Kingdom of Östland
 * Kingdom of Rockhome
 * Söderfjord Jarldoms
 * Empire of Thyatis
 * Kingdom of Vestland
 * Emirates of Ylaruam

Northern Brun

 * Denagoth
 * Frosthaven
 * The Kingdom of Norwold
 * Wendar

Central Brun

 * Hojah
 * Hule
 * Jaibul
 * Sind
 * Slagovich
 * Ulimwengu
 * Yavdlom
 * Zvornik

The Savage Coast

 * Almarron
 * Bellayne
 * Cimarron County
 * Eusdria
 * Gargona
 * Guadalante
 * Herath
 * Land of the Jibaru
 * Land of the Wallara
 * Narvaez
 * Nimmur
 * Orc's Head Peninsula
 * Renardy
 * Robrenn
 * Saragon
 * Texeiras
 * Tortle Tribelands
 * Vilaverde

Western Brun

 * Gombar
 * Suma'a

The Sea of Dread AC1000

 * Thanegioth
 * Undersea
 * Isle of Dread

The Sea of Dawn

 * Isle of Dawn
 * Ochalea
 * Pearl Islands

The Alphatian Sea AC1000

 * The Alphatian Empire
 * Bellissaria
 * Qeodhar
 * Yanifey

Farend Ocean

 * Cestia
 * Everfeed
 * Oceania

Davania

 * Davania
 * Adakkia
 * Amalur lowlands
 * Aryptia
 * Brasol
 * The Diamond Ranges
 * Emerond
 * Ice Peaks
 * Izonda
 * The Jungle Coast
 * Land of the N'djatwa
 * Meghala Kimata
 * Pelatan
 * Serpent Coast
 * Thratia
 * Vulcania
 * Vulture Peninslua

Skothar

 * Esterhold
 * Minaea
 * Thonia

Underworld

 * City of the Stars

Hollow World

 * Antalian Lands
 * Beastman Lands
 * Blacklore Elves
 * Brute Men Lands
 * Gentle Folk
 * Hutaaka Valley
 * Icevale
 * Jennite Holdings
 * Kogolor Dwarf-Lands
 * Krugel Horde
 * Kubitt Valley
 * the Lighthouse
 * Makai Islands
 * Malpheggi Swamp
 * Merry Pirate Seas
 * Milenia
 * Neathar Lands
 * Nithia
 * Oltec Kingdom
 * Schattenalfen Caverns
 * Shahjapur
 * Tanagoro Plainsmen
 * Traldar