Toilets, sinks, tubs and showers all work better if connected to a vent pipe. Kinda like when you put your finger over a straw in a glass of liquid and pull it out. It stays in the straw.
Is this a thing in places that don't have cold winters? I have access to the pipes under my toilets and they go straight down into the basement floor (where they go even deeper to avoid the frost and join the sewer). The pipes are thick (put both hands around them and your fingers won't touch) so the draining water should always have access to air from below which is stopped from coming in the house by the U shaped pipe beneath all sinks and toilets.
It happens in Australia :)
The only thing that doesn't surprise me about that is that it was only a snake, and it wasnt fighting a deadly arachnid of some sort.
Those U shaped pipes are called traps. They are designed to trap enough water in the line to keep sewer gasses from entering your home. Your drain pipe will be connected and dump to a vertical pipe with Y connector, one end of the vertical pipe goes to the sewer and the other up the wall and through the roof. Those pipes you see sticking up through roofs are vent pipes. Your plumbing won’t work well without them.
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