Louise Prockter of Johns Hopkins University, a member of the MESSENGER science team, says she and her colleagues are mystified by a unique feature they're calling ''the Spider,'' seen above.

The formation lies in the middle of the large, 3.8-billion-year-old impact crater called Caloris Basin, and it consists of more than a hundred narrow troughs radiating from a complex central region.

The whole feature is raised off the floor of the basin. The scientists aren't sure whether the small crater near the center of the Spider has anything to do with its odd, raised shape.

One interpretation suggests that the troughs formed when materials in the Caloris Basin floor broke apart.

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