The water of the spring has reached its maximum boiling point. Allah (swt) says:

{ وَإِن يَسْتَغِيثُوا يُغَاثُوا بِمَاءٍ كَالْمُهْلِ يَشْوِي الْوُجُوهَ ۚ بِئْسَ الشَّرَابُ وَسَاءَتْ مُرْتَفَقًا}

(And if they call for relief, they will be relieved with water like murky oil, which scalds [their] faces. Wretched is the drink, and evil is the resting place.) (Al-Kahf: 29)

And their food is not less hot than their water: