I have visited many churches, cathedrals, temples, mosques, and synagogues. If you ask me which one I find the most impressive, it is this one: a church located inside an underground salt mine.
It is neither the most beautiful nor the largest... far from it. But it is a church created by salt miners. The altar, the statues, the bas-reliefs—everything was carved from rock salt.
The Wieliczka Salt Mine in Poland is a UNESCO World Heritage Site that has been in operation since the 13th century. Traditional rock-salt extraction was discontinued in 1996.
While at present it's a famous tourist site, it still produces around 10,000 tons of high-purity evaporated salt annually by pumping and evaporating subterranean brine.
The area opened to the tourists is about 101 meters underground, ending with this level where the church is located, which is only about 1/3 of the total depth of the salt mine.