ISRO scientists think they have found a horizontal uncollapsed lava tube on the moon, 1.7 km long, 360 m wide, and 120 m high (roughly 1 mile x 1200 ft x 400 ft) which could be used as a lunar base by astronauts for inter-planetary missions. [more inside]
The site design is somewhat unfortunate, but The Virtual Cave features lots of photos and information on, well, caves and cave formations. We've all heard of stalagmites and stalactites, but I'd never heard of cave draperies or cave pearls before. Then you've got your helictites, your aragonite, and your splash stalactites (found in lava tubes). And they've got a Show Caves Directory of caves in the United States that are open to the public, with addresses and contact information by state.
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.