I flatted the front wheel going up a climb the other day - I heard the pop and the PSSSSHH, and I could see a geyser of pale "steam" or vapour coming out the puncture.
I swapped out the tube for the spare and continued, but noticed the tube was wet inside the tyre. It hasn't rained on my bike for months.
When I got home I patched the tube, and noticed a lot of dampness around the hole. When inflated it bubbled like a mud pool. The tube also felt squishy and I couldn't push all the air out when stowing it later.
I guessed there was some water inside, so the tube was left hung up with the presta valve at the bottom and unwound.
8 hours later this is what it looked like underneath:
Bigger photo of the pool - this is 10 hours of draining time: Quantity is about 5-6 dessert-spoons full (estimated)
The liquid was runny not viscous, and was clearer on top with the colour sunk to the bottom. The coloured bit looks like mud, or a suspended powder
The tube is labelled as "CST 700c 19-23" and a long number There is no mention of any sealant or puncture resistance to this tube.
Why does my tube have brown liquid in it?
EDIT: I removed the valve core and tried to purge the goop using a rolling motion. It didn't work so I junked the tube. I cut it open to see what was blocking the open valve, and it looked like this:
Not the best photo sorry, but its a jelly-like mass with many black dots like grains of sand. I suspect these are suspended fragments of butyl rubber from the inside of the tube.
TL;DR? Squishy tubes are bad - buy a new one.