Today, the best-surviving features of the Hopewell tradition era are earthwork mounds. Researchers have speculated about their purposes and debate continues. Great geometric earthworks are one of the most impressive Native American monuments throughout American prehistory, and were built by cultures following the Hopewell. Eastern Woodlands mounds typically have various geometric shapes and rise to impressive heights. Some of the gigantic sculpted earthworks, described as effigy mounds, were constructed in the shape of animals, birds, or writhing serpents. Due to considerable evidence and surveys, plus the good condition of the largest surviving mounds, more information can be obtained.