It’s oddly unnerving when archaeologists stumble upon the remains of an ancient civilization, yet have zero clue as to what happened to them. Was it mass immigration or a sudden natural disaster? Or what if aliens were behind it all? In all seriousness, there’s most likely a logical explanation, it’s just that there isn’t much information to go by since it happened so long ago. Take a look at these five ancient civilizations that just up and vanished, without a trace.

1. Rapa Nui

The Rapa Nui were the ancient people who created the famed giant statues on Easter Island. They were of Polynesian descent and researchers are baffled as to how they got there in the first place due to its remote location. So what exactly happened to them? One theory says that starvation caused by excessive resource consumption wiped them out. Others say that the island’s ecosystem was decimated by the overpopulation of rats. Or maybe they traveled to another remote island to start a new settlement. There may be some truth to the last one.

2. Khmer Empire

The Khmer Empire existed from around AD 802 to 1431 in Southeast Asia in what would be the modern countries of Thailand, Vietnam, Cambodia and Laos. The Empire’s culture was that of a mixed one of Hindus and Buddhists following centuries of warfare. But over time, their population started declining and then poof! Gone, without a trace. Some theories point to changes in weather conditions which changed their access to rainwater; others say that the Khmer people were slowly overtaken by the Thai people. Either way you slice it they’re not sure what happened.

3. Minoans

The Minoans hail from the Greek island of Crete during the Bronze Age between 3000 and 1000 BC. They were even mentioned in Ancient Egyptians hieroglyphs and were noted for their impressive technologies and arts. Then their numbers started dwindling until they vanished for good. Some people say they may have been ravaged by a volcanic eruption in the Santorini islands near Crete. Several centuries after their disappearance, the Greek historian Herodotus wrote that they were wiped out by plagues and diseases.

4. Olmecs

Then there’s the Olmecs (if that’s what they even called themselves because that’s what the Aztecs referred to them as). There’s no physical evidence that they ever existed, not even bones. We don’t even know what their language was like. The only proof of their existence were the massive stone head statues that resembled the ones on Easter Island, measuring 10 feet in height and weighing 8 tons. It is theorized that the Olmecs exited the world stage around 400 BC and that their bones were eroded by the humid Mesoamerican climate.

5. Anasazi

The Anasazi were a Native American tribe who built massive housing structures into the cliffsides. These structures were often multiple stories tall and were accessed via windows using a ladder. They also offered a much-needed advantage when it came to fending off invading tribes by climbing to the top of the structure and firing down onto their enemy. According to some surviving Native American tribes, the Anasazi never truly disappeared, their population grew to the point of breaking off into smaller groups and that certain tribes are their descendants.