The Pool of (add the name of the God or Goddess of your choice) has been holy to humans since humans first looked up at the sky and felt that there must be Something up there. In the earliest times it was sacred to Inana, and when her religion dwindled to cult status and then to nothing at all, the priests of another deity took it over. It is thought to have changed hands in this way at least seven times, and maybe more.
In time a temple of fine marble was built over it and worshippers were encouraged to wade along the edges of it and to drop offerings. As a result, the pool, which is only ten feet deep at the deepest part, is full of copper,silver and golden coins, and even here and there jewels, that have been dropped by anyone from a poor person desperate for divine blessing to the richest nobles. When nesscessary the priests can take as much as is needed for keeping themselves alive or for repairing or extending the temple.
Should anybody try to steal from the pool, the water at once heats up dramatically as a warning;should the thief not at once stop theact of sacriliege and return what is stolen, it heats to boiling point, scalding or killing the thief depending on how deep the thief is immersed in the pool at the time. It will do this even to a priest or priestess unless the money is being used for a proper purpose. Should anyone try and take the area by force, the pool will do much the same thing, presenting a boiling, bubbling threat to any attacker. As a result soldiers and iconoclasts tend to leave it alone.
It can be used for other things too;
The Ordeal by Water-Somebody accused of a crime will publicly take a holy oath that he or she did not commit the crime they are accused of and wade waist-deep through the pool. If they have lied the pool will wait until the liar is far from the edge, and then boil itself, badly burning the criminal. Should the oath be true, it remains cool, proving the person's innocence.
Dreams-someone who manages to fall asleep in the warm waters of the pool (and avoid drowning in the process) has a high chance of prophetic dreams. (the GM should decide what the dream is and how accurate the prophecy is.)
Long life-Whoever drinks from the pool will be granted an extra day without ageing. The catch is, that when taken more then five feet from the pool, the water loses this power, so to stay immortal, one would have to stay close enough to the pool to drink from it each day. Such is the limit,and the price, of immortality.
What the pool really is...
Link to the Gods-The pool is indeed a link to the Gods of both the Air and the Earth. The offerings here strengthen them, whilst prayers here have a better chance of being replied to then anywhere else.
Elementary, Dr Watson- The pool is where an ancient and by now vastly powerful Water Elemental lives. It loves being worshipped and hates thieves and liars, and whilst it could crawl from the pool, revealing it's true nature, it has been pampered for so long that it does not want to.
The Wizard of Oz-The pool is a sci-fi construct/spaceship/listening device/all three, that can heat it's waters or cool them with hidden tech and influence people's dreams in some way, and it's waters have nanobots in them that heal but die when the person or animal goes more then five feet away from it.
Notes-I wanted this pool to be useable in any world that has a religion in it, so that's why the name of the Pool was left blank. And I put in a plot hook so that it can even be used in a sci-fi game if the GM feels like it.