HOW I HACKED THE WATER PIPE
This is what I used to hack a water pipe:
Gardena 3,5 Bar pump
20 Gramms of blue food coloring
First I cleaned and disinfected the tub, then I filled it with cold water. The tub you can see in the picture holds 150 liters.
Next I dissolved 20 grammes of food coloring in warm water and poured it into the tub.
I connected the hose of the pump to the cold water outlet above the valve and then opened the valve.
The pressure of the fresh water is 1,6 bar, the Gardena pump produces 3,5 bar. After opening the valve to the fresh water outlet, I turned the pump on and pumped the blue water back into the water connection.
This experiment took place on a trip in Europe, at 3 a.m. in the morning. I was quite sure that, at that time, no water was being used in the house.
It took almost an hour to pump the water back, due to the pressure of 1,6 bar that was already in the pipe. I had put aside some water in buckets before starting and eventually pumped some uncolored water into the water tap to cover my tracks.
I didn't have the chance to take pictures in one of the neighboring flats since I left in the afternoon and did not want to raise too much attention, but from what I heard, the water kept coming light blue from the taps in the entire house the whole day through.
The only place I did get a chance to check on was the toilet of a shop in the basement.
I was really surprised to find it relatively easy to undertake this manipulation, and I am afraid that, with more water available and a stronger pump, one could easily reach the main pipe too.