Here are some of the seriously extreme ones I've seen in the past:
 
 
Erta Ale Volcano  (GCB5D7)  Ethiopia
 
http://www.geocaching.com/seek/cache_details.aspx?guid=1dbe8a3d-1d7f-4d05-93cb-23421a06bcaa

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
 
http://www.geocaching.com/seek/cache_details.aspx?guid=adc42ce8-1aa0-42c7-b00e-4488a098105d
 
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
 
http://www.geocaching.com/seek/cache_details.aspx?guid=75b571d8-27dd-40ba-a956-f5b0614d7404
 
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
 
http://www.geocaching.com/seek/cache_details.aspx?guid=b4ed7418-4178-414d-bb09-383e92a475aa
 
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
 
http://www.geocaching.com/seek/cache_details.aspx?guid=2be7d04c-d99e-491d-abcc-2c3b5abd77d3
 
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
 
 

		
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