Close to an explosive situation

The pressure obviously built up overnight as fermentation commenced.  I sure didn't expect to find this with 2 gallons of headspace in the 5 gallon carboy (that is a 3 gallon batch for those not good at math).  I guess the yeast was healthy and obviously began with an active roar.

The airlock had filled with krausen, blocking it up.  I've heard of this happening but this is a first to experience it.  I pulled it out of the top stopper to an audible release of pressure.  I can only imagine what a few more hours (or minutes) could have done.  Boom, with krausen and wort splashing all over the place.

Lesson of the day:  Use a blow-off tube for initial and active fermentation.  Don't assume you have enough head space.

Here is the photo of it now happily fermenting away.