As we learned on yesterday’s daf , the Sages of the Mishna forbade the people of Tiberias from heating up cold water by means of a pipe that was placed in the hot-springs in that city. The Gemara on today’s daf tries to ascertain why that was prohibited, suggesting that the hot-springs are considered heated by fire. The Gemara teaches:
The Rabbis said the following to Rabbi Yose: Wasn’t the incident involving the people of Tiberias with derivatives of the sun, as the hot springs of Tiberias are not heated by fire, and nevertheless the Sages prohibited them from using the water? Rabbi Yose said to them: That is not so. That incident involved derivatives of fire, as the hot springs of Tiberias are hot because they pass over the entrance to Gehenna.They are heated by hellfire, which is a bona fide underground fire. That is not the case with derivatives of the sun, which are not heated by fire at all.
The Sages who say that the hot springs of Tiberias are heated by the sun mean that the water is not heated by fire. Any source of heat other than fire is comparable to the sun in the sense that cooking with it is significantly different from cooking with fire and should be permitted. Rabbi Yose, on the other hand, believes that the hot springs are heated by the fire that arises from beneath the ground, called here hellfire. The legal status of the hot springs is analogous to water heated by fire although their fire was not lit by human hand.
In fact, the Tiberias hot-springs, near the Sea of the Galilee, contain geothermally heated groundwater that is at a constant temperature of 60 degrees Celsius (140 degrees Fahrenheit). In general, the temperature of rocks within the earth increases with depth. When water percolates deeply enough into the crust, it will be heated as it comes into contact with hot rocks. Much of the Earth’s internal heat is produced by decay of naturally radioactive elements, which is not “fire” in the normal sense of the term.