Stonehenge in England is grudgingly yielding more of its secrets, this time from deep underground in a detailed 3D map of the monument and its surroundings, researchers say.

Using instruments that scanned around the iconic circle of stones to depths of almost 10 feet, scientists say they've discovered Stonehenge did not stand by itself but was surrounded by at least 17 other shrines in its vicinity.

Burial mounds, ritual shrines and hidden chapels were revealed by ground-penetrating radar and other instruments used to survey 4.6 square miles around the central circle of stones that is Stonehenge.

"Despite Stonehenge being the most iconic of all prehistoric monuments and occupying one of the richest archaeological landscapes in the world, much of this landscape in effect remains terra incognita," says Vince Gaffney, leader of Birmingham University's Stonehenge Hidden Landscapes Project.

"In the past we had this idea that Stonehenge was standing in splendid isolation, but it wasn't ... it's absolutely huge."

A major find is one of the most ancient structures in the area; a burial mound built on top of a 6,000-year-old timber hall believed to serve as a "house of the dead," where bodies of the dead were ritually stripped of their flesh and the skeletons then disassembled.

The building resembles similar but older buildings found on the European continent, the researchers said.

"It is around 300 square meters and slightly trapezoidal, which is interesting because in the same period on the continent, about 100 to 200 years earlier, we also find this type of trapezoidal building related to megaliths [giant stones]," says Wolfgang Neubauer, director of the Ludwig Boltzmann Institute in Vienna, which also took part in the research.

The 3D map, which took 4 years of scanning and analysis of terabytes of data to create, is a valuable research tool even if it shouldn't be compared to a modern road map, Gaffney says.

"People look at this map and say, 'oh, there was a plan!' But actually, I don't think there was a plan; it's something that emerged," he says.

Stonehenge and its surrounding are a landscape that was reshaped over generations and hundreds of years after the first stones of the central monument were laid down around 3,100 B.C., he says.

"What you're seeing is a perpetual re-emphasis on this piece of land."