Figure 1.  Chicxulub crater, Yucatan peninsula, Mexico.  Remains of the meteorite impact which 65 million years ago drove the dinosaurs into extinction.  The white line cutting across the center of the crater (and figure) is the northwest Yucatan coastline.  Patterns of white dots indicate clustering of “cenotes”, water-filled sinkholes in the limestone “karst” bedrock.  As the indicated article in Nature lays out, this “image” of the ancient crater, stripped of up to a kilometer of enveloping sediments, was produced by measuring the “horizontal gradient of the Bouguer gravity anomaly.”  As it says, “Areas lacking gravity data appear blurred,” while “Darker shading corresponds to greater gradient magnitude.  The technique's sensitivity is illustrated by the detection of part of the Ticul fault [white diagonal lines near bottom of figure] that has only ~100 m [about 330 feet] of vertical displacement at the surface.” Thanks to Nature article co-author and preparer of the figures Mark Pilkington (of the Geological Survey of Canada) and to the Nature Publishing Group for their kind permission to use the figure. 1

It's well accepted among scientists nowadays that the dinosaurs — along with three-quarters of all species on earth, land and sea — disappeared in the aftermath of the cataclysmic collision of a sizable asteroid or comet impacting the earth some 65 million years ago.  Though the huge, multi-ringed, some 180 kilometers (112 miles) in diameter crater resulting from that cosmic trainwreck was found over a decade ago — buried at the northwestern tip of the Yucatan peninsula in Mexico, beneath up to a kilometer of sediments — it's apparently invisible as far as any visible effect on the surface topography of that part of the Yucatan (which basically is about as flat as a pancake) is concerned.

The deeply buried crater's seeming irrelevance to affairs on the surface is more apparent than real, however.  The porous limestone (so-called “karst”) basement of the Yucatan is pierced by a plenitude of water-filled sinkholes, known locally as “cenotes”.  These cenotes fall into patterns (easily visible on detailed road maps, for example) which, it turns out, congregate along the buried outer wall of the stupendous concentric-ringed prehistoric crater.

(Thus, one can swim in a pool lying atop the old dinosaur-killer crater rim.  A fond remembrance: rather like the Pantheon in Rome, one might say, the underground cenote raised a vast dome vaulting through the rock ceiling into daylight for only a brief circle, through which sunlight poured to blaze in the otherwise dark, quiet pool, down the roof and along the sides of which stalactite and stalagmite columns marched….)

As A. R. Hildebrand and his colleagues (authors of the paper in Nature “Size and structure of the Chicxulub crater revealed by horizontal gravity gradients and cenotes” that is considered in this piece) describe it: ¹

The cenote ring corresponds to (and was presumably created by) a zone of enhanced groundwater flow, as evidenced by a coincident low in the groundwater surface and the presence of freshwater springs where the cenote ring intersects the coast.  High hydraulic conductivity is also indicated in the surrounding near-surface rocks by low hydraulic gradients and widespread response to tides.  […]  The ring consists mostly of water- and reed-filled cenotes of larger size (typically up to ~300 m [~1,000 feet] diameter) than the cenotes and dry karst pits found elsewhere on the peninsula.  On the crater's east side, the size of the main ring's constituent cenotes allows it to be distinguished from the widespread karst features exterior to the crater.  The main cenote ring is up to ~5 km [~3 miles] wide, and corresponds to a topographic low of up to ~5 m [~16 feet] over some of its length.  Much of the permeability in karst terrains like the Yucatán peninsula is due to fracture systems, so it has been assumed that the cenote ring corresponds to a zone of enhanced fracturing.  Although fractures are not directly observable along the cenote ring, we presume that a fracture system of ≤5 km width with orientations preferentially parallel to the underlying crater structure created the permeability that lead to the ring's formation.  A zone of enhanced fracturing would also allow preferential erosion to create the topographic low associated with the ring.

The authors also note that, “The edges of the crater (and associated gravity-gradient features) correspond to bends in the coastline, and interior gradient features sometimes correlate with rocky points along the coastline.”

Beyond those surficial physical consequences of the subterranean crater's presence, the team assembling this marvelous figure and accompanying article, in effect — “merely” by careful measurement of seemingly trivial changes in gravity — have stripped the veil off this vast interred sepulchre, revealing the blasted hole in all its ruined glory!  Kudos to the group for putting together this dramatic portrait — which isn't an image at all in the sense of a photograph, but might as well be for the clarity of the scene it presents.

The authors interpret the spectacular visage of the ancient crater: ¹

Figure 1 reveals a striking circular structure centred near the Yucatán shoreline at ~89.57° N, 21.29° W.  At least six concentric gradient features occur between ~10 and ~90 km [between ~6 and ~56 miles] radius.  These features are probably most distinct in the southwest owing to denser sampling of the gravity field.  Truncations of the gradient features often correspond to gaps in survey coverage.  […]  We interpret the outer four gradient maxima (at ~55 to ~90 km radii) to represent concentric faults in the crater's zone of slumping, as are also revealed by seismic reflection data.  The inner two maxima probably represent the outer margins of the central uplift (at 20-25 km radius) and the peak ring and/or collapsed transient cavity (at 40-45 km radius).  Radial gradients in the southwestern quadrant over the inferred ~40-km-diameter central uplift may represent structural ‘puckering’ as revealed at eroded terrestrial craters.  Gradient features related to regional gravity highs and lows are visible outside the crater, but no concentric gradient features are apparent at radial distances >90 km [>56 miles].  Note that the crater's outer gradient and karst features are linear near the northern and tangential part of the Ticul fault […].

Hildebrand et al. acknowledge that, “the peripheral strong gradient features are truncated or diverted for the northern third of the crater.  Magnetic and seismic data confirm that a completely circular basin and impact structure is present, and some weak circular structure appears in the gravity data north of the truncation of the peripheral gradient features, but the cause of the truncation of Chicxulub's gravity expression remains to be understood.”
 

Here's the Abstract from A. R. Hildebrand and colleagues' Nature article: ¹

It is now widely believed that a large impact occurred on the Earth at the end of the Cretaceous period, and that the buried Chicxulub structure in Yucatán, Mexico, is the resulting crater.  Knowledge of the size and internal structure of the Chicxulub crater is necessary for quantifying the effects of the impact on the Cretaceous environment.  Although much information bearing on the crater's structure is available, diameter estimates range from 170 to 300 km (refs 1-7), corresponding to an order of magnitude variation in impact energy.  Here we show the diameter of the crater to be ~180 km [about 112 miles] by examining the horizontal gradient of the Bouguer gravity anomaly over the structure.  This size is confirmed by the distribution of karst features in the Yucatan region (mainly water-filled sinkholes, known as cenotes).  The coincidence of cenotes and peripheral gravity-gradient maxima suggests that cenote formation is closely related to the presence of slump faults near the crater rim.
 

¹ A. R. Hildebrand, M. Pilkington, M. Connors, C. Ortiz-Aleman, and R. E. Chavez, “Size and structure of the Chicxulub crater revealed by horizontal gravity gradients and cenotes,” Nature, Vol. 376, Issue No. 6539 (dated 1995-08-03), pp. 415-417 [doi:10.1038/376415a0].  Requires pay-per-view for full text.

² H. J. Melosh, “Around and around we go,” Nature, Vol. 376, Issue No. 6539 (dated 1995-08-03), p. 387.  Accompanying news piece; requires pay-per-view for full text.
 

UPDATE:  2004-01-19 19:00 UT:  Substantial rewording and material added from Hildebrand et al.

UPDATE:  2005-07-24 06:00 UT:  Compression on crater image decreased.