Yesterday we mentioned the launch of a little-known game called Terraria . It appeared on Steam with no marketing campaign, and no real build up beyond a link on Twitter from Minecraft creator notch . Right now, it's the sixth most played game on Steam, ahead of Portal 2 and Team Fortress 2, and is the second best selling game on Steam, lingering just behind The Witcher 2.

The Terraria forums noted that at its first day peak, over 17,121 players were playing the game at the same time. They estimate that the game has sold over 50,000 copies (at $9.99 a go). That figure's set to rise over the coming days as word spreads.

So what is Terraria? The words "two-dimensional Minecraft" go a long way to summing it up. The fully destructible, randomly generated worlds can be mined for materials that can be crafted into tools. The big differences between Terraria and Minecraft, beyond the missing dimension, is the madness of the gear that you can craft. From laser weapons to magical swords and grappling hooks, there are dozens of items to construct.

Then, once fully kitted out you can expect to encounter some much more threatening monsters than you'd meet in Minecraft, including giant demonic eyeballs and a snake called The Eater of Worlds.

There's also a multiplayer mode which supports as many players as the server you're playing on can handle. See the trailer below for a taste of the madness that ensues when more than a couple of humans get involved. Note also the amazing whirly-armed attack animation. Are you playing Terraria? Let us know your thoughts in the comments.