All across Texas, the bones of history lie in watery graves. From the ribs of sunken ships to the grave sites of prehistoric Texans, uncounted treasures abound beneath the surface of rivers and lakes. For state archaeologists, these sites are untapped treasures — hard to reach but relatively protected.

But now, with the state in the grip of devastating drought, such sites are emerging from receding waters and — for the first time in years, experts worry — becoming vulnerable to looters and vandals.

Since midsummer, the Texas Historical Commission, which oversees such locations, has on average learned of a newly exposed site each month, said Pat Mercado-Allinger, the agency's archaeology director.

Among the sites are four cemeteries, including an apparent slave burial ground in Navarro County, southeast of Dallas. In Central Texas, fishermen recovered a human skull thought to be thousands of years old.

An unspecified number of additional sites have emerged from waters overseen by the Lower Colorado River Authority. An agency spokeswoman refused to discuss details, saying that even divulging the number of newly exposed sites could induce the unscrupulous to search out and pilfer them....