Lager yeast is very sulfuric, what did you brew last in that carboy?
You likely got some wild yeast and some bacteria.
Don't taste it until it runs its course, bacteria can be dangerous.
Once the yeast take over, drop the PH, and raise the alcohol level the bacteria will settle down or die out.
Be prepared to let this sit for several months, wild yeast does not attenuate very well.
BYO and Brewers TV have done some interesting specials on spontaneous fermentation that you may want to take a look at.
It will turn out very different than what is intended but I would stick with it... for science!
If it turns out well, save a sample of the yeast for future use.