The mystery surrounding Stonehenge has suddenly deepened -- literally. A first-of-its-kind study suggests that 15 previously undiscovered or poorly understood monuments lie hidden under the ancient stone monument and its surroundings.

For the study, researchers used a variety of techniques -- including ground-penetrating radar and 3D laser scanning -- to create a highly detailed subsurface map of the entire area. According to a release from the Ludwig Boltzmann Institute for Archaeological Prospection and Virtual Archaeology, one of the partners in the study, the technologies are notable for being much less destructive than traditional, digging-based exploratory techniques.

Known as "The Stonehenge Hidden Landscape Project," the four-year effort suggests that there was more going on in the area than previously thought -- as evidenced by all the newly identified monuments.

One of the new finds is an ancient trough that bisects an East-West ditch known as a "Cursus," Prof. Vince Gaffney, an archaeologist at the University of Birmingham in England and one of the scientists behind the project, told The Smithsonian.

Gaffney said he believes that the Cursus monument aligns with the sunrise on the Spring and Fall equinoxes, and that the newly discovered trough could have been a means for people to ceremonially process toward the center of Stonehenge to the south.

The trough and the other newfound monuments have "absolutely transformed" how archeologists view the area, Gaggney said. Yet "until you dig holes," he acknowledged, "you just don’t know what you’ve got."

The new survey builds on findings from last October indicating that the area around Stonehenge is the oldest continually occupied region in Britain. The scientists behind that research said that the land there may have been occupied since 8820 B.C.