Board Thread:New on Terraria Wiki/@comment-24704180-20140319182759/@comment-24133975-20140319184015

Make sure you destroy all the Demon Altars you can find. For every Demon Altar you destory a new Hardmode ore spawns in.

Example: First Demon altar = Cobalt: Second Demon Altar = Mythril: Third Demon Altar = Adamantite

Each of these Ores will spawn all over the world whenever you break the Altars.

The more Altars you break the more Ore spawns.

You can find more info on this at the Demon Altar page.

Hope this helps! :D