Why is there a tower of glass in my lake?

While exploring I've found a tower of glass blocks coming out of a lake in Minecraft.

As far as I can tell they go up to the top limit of the world.

It also goes down in to the ground, as you can see by this quick test dig on the lake bed:

Why is it there?
Bug or feature?
Does it mean anything specific?
etc

For Info: I'm using Beta 1.9 Prerelease 3, and have only ever used 1.9pre3 (I'm a very new MC player, and thought I'd start with the "latest" version), so this can't be an artifact from map generated by the old version, for example.

Also I have no mods, tweaks, etc in place - the game is completely stock.

Solution 1:

In 1.9 pre-3 some debug tools have been left on. One of which is glass towers coming out of the top of strongholds:

Debug Features:

• 1x1 glass pillars to spot Strongholds

If you follow this glass spire down, you should hit a stronghold.