NEWS AND VIEWS

10 melts when 'squeezed' by increasing the superconductors . Similar physics causes mise of elite power, the abandonment of external field. quantum solids with long-range inter­ many urban centres, and important demo­ Although more detailed theories are actions to melt when squeezed, with zero­ graphic and economic shifts from the 2 available , a qualitative understanding re­ point energy replacing entropy in the southern to the northern lowlands. Con­ sults if thermal fluctuations in the crystal above arguments. tinued dryness for two centuries, as the are modelled by a single vortex filament in Related arguments2 show that the Abri­ the cage formed by the repulsive inter­ kosov vortex crystal must also melt at very 9 actions with its neighbours . Both the low densities and fields, along the re­ flexibility of the line and the range of entrant dashed curve shown in the figure. 1 interactions are important. The flexibility The physics of low-field melting, the field 2 determines how far along the average field dependence of vortex configurations at direction the line wanders before it is very low temperatures and the behaviour 4• Coring deflected back by the bars of its cage. The above the high-field critical point are site denser the vortex array, the more these inviting subjects for future investigation. collisions increase the en tropic cost of the crystalline phase. When the vortex in­ David R. Nelson is in the Lyman Laboratory teractions are relatively long range (as is of Physics, Harvard University, Cambridge, usually the case), squeezing triggers a Massachusetts 02138, USA. melting transition when the density is high 1. Zeldov. E. eta/. Nature315. 373-376 (1995). enough that the extra entropy of vortex 2. Blatter. G. eta/. Rev. mod. Phys. 66, 1125 (1994). braiding in an entangled flux liquid phase 3. Brezin, E. , Nelson, D. R. & Thiaville. A. Phys. Rev. 831, outweighs the potential energy cost. A 7124--7131(1985). 4. Safar. H. eta/. Phys. Rev. Lett. 69, 824 (1992). density reduction on freezing might also 5. Charalambous, M. eta/. Phys. Rev. 845, 45 (1992). be expected when flexible nematic poly­ 6 . Jiang. W. Phys . Rev. Lett. 14, 1438-1441 (1994). 7. Safar, H. eta/. Phys. Rev. Lett. 71. 436-439 (1993). mers with low salt concentration crystal­ 8 . Fendrich,J. A. eta/. Phys. Rev. Lett. 14, 1210 (1995). iloQ k_m lize into the hexagonal columnar phase. 9. Nelson. D. R. in T/Je Vortex State (eds 8ontemps, N. et ) ' a/.) 41-61 (Kiuwer, Dordrecht, 1994). --­ DNA molecules, for example, have in­ 10. Podgornik, R. & Parsegian, V. A. Macromolecules 23, The Yucatan peninsula showing the coring site teractions quite similar to vortices in 2266 (1990). (Lake Chichancanab) of Hodell et a/2 and ARCHAEOLOGY Maya sites mentioned in the text. results of Hodel! et al. suggest, might also have inhibited cultural and demographic Drought and decline recovery in the previously degraded parts of the southern lowlands. Jeremy A. Sabloff The research by Hodel! and colleagues contributes to scholarly attempts to AROUND AD 800, something happened to triggers, many of which have yet to be understand why and how the Classic Maya the Classic . The rela­ identified, for the collapse of individual successfully adapted to their tropical tively sudden decline seen at many sites, cities may have varied within the overall environment for so many centuries, over­ particularly in the southern lowlands matrix of demographic and environmental coming numerous problems and reaching (which lie in modern-day , pressures. population levels that far exceed modern , and ), has gar­ The new research by Hodel!, Curtis and ones in the same area, and why they 2 nered considerable scholarly and lay in­ Brenner , adds another important compo­ ultimately failed. Their research also 1 terest over the years • On page 391 of this nent to the model that archaeologists are raises an anthropological question that is issue, Hodell and co-workers2 supply currently building to help explain the relevant not only to the ancient Maya but fresh evidence that may point the finger at upheavals of the late eighth century and to the contemporary world as well, and is climate changes in the region as one key early ninth century AD in the Maya low- certainly deserving of continued attention factor in this decline. 1-> nrk As some of the factors were present -namely, how severe do internal stresses In the past two decades, as archaeol­ well before AD 800 and the ancient Maya in a civilization have to become before ogical field research in the Maya area has suffered previous setbacks in their relatively minor climate shifts can trigger 6 boomed and the deciphering of Maya development , one unanswered question widespread cultural collapse? D hieroglyphic texts has generated new his­ has been why these factors had such a torical information, archaeological under­ cataclysmic effect around AD 800. What Jeremy A. Sabloff is at the University of standing of the 'collapse' of Classic Maya Hodell et al. find, by examining sediment Pennsylvania Museum of Archaeology and 5 civilization has grown3- . Interrelated fac­ cores from Lake Chichancanab in the Anthropology, Philadelphia, Pennsylvania tors including population growth, en­ northern lowlands of the Yucatan pen­ 19104-6324, USA. vironmental degradation and intercity insula (see map), is that a period of conflict have been implicated in the pro­ increased climatic dryness set in around 1. Culbert, T. P. The Classic Maya Collapse (Univ. New cess of decline . In addition, the concur­ AD 800. Mexico Press, Albuquerque, 1973). 2. Hodell. D. A., Curtis. J. H. & Brenner, M. Nature 375. rent florescence of cities in the northern To a civilization facing a number of 391- 394 (1995). lowlands such as , Kabah, and stresses both internal and external, the 3. Sablolf. J. A. The New Archaeology and the Ancient Maya (Freeman. New York, 1990). and the lack of decline at scarcity of water could have greatly in­ 4. Sablolf,J. A. & Henderson. J. S. (eds) Lowland Maya certain sites in the southern lowlands creased the vulnerability of numerous Civilization in the Eighth Century AD (Dumbarton Oaks, with advantageous riverside locations Classic Maya cities, especially in the Washington DC, 1993). 5. Sharer, R.J. The Ancient Maya , 5th Edn (Stanford Univ. can now be better appreciated, as these southern lowlands where the strains were Press. 1994). centres are situated either in zones with greatest. In particular, with the land­ 6. Marcus, J. in Lowland Maya Civilization in the Eighth CenturyAo(edsSabloff,J. A &Henderson, J. S.) 111- 184 previously low populations or in loca­ scapes around manl: Maya centres already (Dumbarton Oaks , Washington DC. 1993). 8 tions that provided economic buffers under heavy stress · , the arrival of a drier 7. Rue, D. J. Nature326. 285--286 (1987). 8. Rice. D. S. in Lowland Maya Civilization in the Eighth against severe environmental strains. It regime would have exacerbated the peril­ Century AD (edsSablofU. A. & Henderson, J. S.) 11-63 also has become apparent that the exact ous situation, ultimately causing the de- (Dumbarton Oaks, Washington DC, 1993). NATURE · VOL 375 · 1 JUNE 1995 357