Last chance to see: Video of Mexico's Naica Cave of Crystals (Previously, and previously.) [more inside]
225° F (105° C) heat index. Easily impaled on 55-ton jagged crystals that are 36 feet long ... made out of drywall (kinda). "Most cameras with moving parts and tape mechanisms simply will not work."
The gypsum crystals in the Cave of Crystals at the Naica mine in Chihuahua, Mexico, are some of the largest and most spectacular in the world. [Last link is a .wmv]
Mystery of 'chirping' pyramid decoded: "A theory that the ancient Mayans built their pyramids to act as giant resonators to produce strange and evocative echoes has been supported by a team of Belgian scientists." Others are not so sure... Coincidence, or engineering? Did the designers of El Castillo pyramid cannily build in a sound effect that mimics the warble of the sacred quetzal bird? Listen for yourself, with the .wav file (first set is the real bird, the second is the pyramid) featured in this Acoustical Society of America page. I prefer to think it's deliberate; after all, it's possible that early man was experimenting with cave acoustics to to create sound-enhanced rock art (there are sound samples for this included here - unfortunately a Geocities site). Also of interest, the BBC programme "Acoustic Shadows" (requires RealPlayer - *heavy sigh*).
50 foot long single spar crystals found in a Mexican cave 1,000 feet below the surface! Smithsonian has links to other related sites. This one has pictues. More pictures can be found in the April 2002 print issue of Smithsonian.