The bottom of Lake Altaussee has an underwater forest with approximately 150 trees that are standing upright. Most of the trees were identified as fir. Applying dendrochronological methods, the time of death can be determined quite accurately. The outermost growth rings of the trees examined date back to around 860 AD.
While soil, gravel and clay trapped in the root zone of the underwater trees differ from the sediment material found elsewhere in the lake bed, they most closely resemble the material from the western mountain slopes. Therefore, the trees most likely originated from the surrounding mountain slopes, and slipped down into what we know as the lake bed today. What caused this event is still a mystery, but it was probably associated with a major c limatic event or earthquake.