Here are some of the seriously extreme ones I've seen in the past:

 

 

Erta Ale Volcano  (GCB5D7)  Ethiopia

 

The description alone gives me the willies. "To lean over the crater without protection is out of the question. Just to have a quick look, one must position gloved hands in front of the face. Aluminium, fire-proof clothing Nomex is a must. To protect the eyes, a sheet of polished gold must be used; it is only few microns thick and so see-through."

 

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Rainbow Hydrothermal Vents  (GCG822)   Off the coast of Portugal

 

 

Placed by Richard Garriott, AKA Lord British, the man behind the "Ultima" and "Ultima Online" series of computer games, with aid of a Russian Mir submersible 2300 meters underwater.  A first find is still available.  :)

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Tube Torcher  (GC3CC9)    North Carolina

 

 

A must-do cache in North Carolina.  Looks like a favorite test of geocaching prowess.

 

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Finishing up with the most involved caches I have personally done :

 

Snakes and Ladders (GCJG08)   Escondido, California

 

 

A monstrosity of a multi-cache.  The number of waypoints, depending on how many wrong turns you make, can vary between 15 to 30.   The waypoints are also definitely of the "outside the box" kind.

 

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And a shout out to Tucson's cruelest puzzle cache:

 

Giggles  (GCHB6V)   Tucson, Arizona

 

 

Even if you never get a chance to hit the Old Pueblo to attempt this one, reading the logs are worth the price of admission.

 

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Dave, The Cow Spots