Install a pressure reducing valve
The boiling point of a liquid is defined as when the vapour pressure of that liquid is equal to the atmospheric pressure. On mountains, the air pressure is lower than at sea level, so it follows that the boiling point of water is lower there.
The gas pressure in the flask is lower than the atmospheric pressure when the water level is higher inside than outside the flask.
First understand that the boiling of water occurs when the energy of water molecules is equal to the vapour pressure... (i.e energy of water is equal to the external pressure due to the atmosphere)... above sea level the atmospheric pressure is lower hence low energy is required... hence water boil at low temperature...
In general, the higher the altitude, the lower the pressure, and the lower the pressure the lower the boiling point.
Because in the mountaintop the pressure is lower than in the sea level; I think.
Due to lower atmospheric pressure at altitude, water will boil at lower temperature.
As you get higher above sea level, air pressure decreases. Water boils at a lower temperature when the air pressure is lower. When the water temperature is lower, pasta and anything else takes longer to cook.
Because the pressure high up in the moutains is lower than on sea level.
At lower pressure boiling points decrease. At high elevations, such as in mountains the atmospheric pressure is lower than at seas level.
I think that the temperature required to boil water is lower at high altitudes (where the atmospheric pressure is lower) than at sea level.
a borehole into the lower stratum below the water level so that pressure forces the water upwards
You are probably above sea level so that the atmospheric pressure is lower than sea level pressure. Water boils at lower temperatures as the pressure is lowered. The other possibility is that the thermometer is not calibrated correctly, but I'd go with the atmospheric pressure is below sea level pressure.