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Thorium Ore is mined from various Thorium veins, with a minimum mining skill of 230.

This rare metal combines the weight of lead with the strength of steel. Orcs prize it for weapons because the extra weight allows a skilled user to strike with more force. Thorium armor is amazingly heavy and strong-only adamantine pierces it with any degree of reliability.^{WoWRPG 194}

Sources

Small Thorium Veins

Small Thorium Veins can be mined at skill level 200, and produce 1 to 3 Thorium Ore. These appear in areas where the average mob level is between approximately 45 and 57.

Small Thorium veins are most commonly found in Blasted Lands, Un'Goro Crater, and Felwood, but are found elsewhere as well.

Rich Thorium Veins

Rich Thorium Veins can be mined at skill level 215, and produce 3 to 5 Thorium Ore. These appear in areas of Azeroth where the average mob level is above approximately 55.

Rich Thorium Veins are most commonly found in Blasted Lands (in the Tainted Forest area), Eastern Plaguelands, Un'Goro Crater and Winterspring.

Oozing Thorium Veins

Ooze Covered Rich Thorium Vein and Ooze Covered Thorium Vein are found in Silithid hives. These are essentially the same as Rich and Small Thorium Veins, respectively, but with a different appearance.

These veins are most commonly found in Silithus and Un'Goro Crater.