 Welcome to Spotlight Advanced. I'm Colin Louther. And I'm Alice Irizari. Spotlight uses a special English method of broadcasting. It is easier for people to understand, no matter where in the world they live. In 2008, Dr. Heinrich Frank discovered something strange. He was driving home from his work at the Federal University of Rio Grande do Sol. He noticed a hole in a building area he drove by. He was a geologist, a person who studies the earth and its minerals and structures. The tunnel interested him. It looked different from other tunnels in the area. So, several days later, he went to investigate. Inside, the tunnel was different from anything he had seen before. Most caves form when water flows underground. They do not go in a straight line. But this tunnel did. It went back into the hill for 15 meters. And, strangest of all, there were marks on the walls. They looked like claw marks made by a huge animal. When Frank left the cave, he did not know what he had found. Soon, he understood that he had discovered his first paleoburro. These ancient tunnels were made by prehistoric animals over 10,000 years ago. Today's Spotlight is on Paleoburros. We know about most ancient animals through their fossils. The study of former life on the earth as preserved in fossils is called paleontology. Usually, fossils are the bones of an ancient animal. These bones have turned to stone over time. Sometimes, scientists are able to find other remains. Rarely, scientists may find animal droppings and even footprints. But these paleoburros, or ancient burros, are new in the history of the paleontology. These were an animal's home. A burro is a hole an animal digs in the ground as a living space or shelter. Through these burros, scientists can do more than picturing how the animals looked. They can study how an animal may have lived and worked. They can know more about the animal's social relationships. Each type of paleoburro would have been dug by a different kind of animal. All of these burro tunnels are very large. They are dug out of solid rock. It would take many animals working together a long time to dig a burro. One paleoburro found in the Amazon is over 100 meters long and 2 meters high. Mina's Jerez is a state in Brazil. There, scientists have found a paleoburro with 6 tunnels, 40 meters long. This burro has a large cave in the middle. The cave is 4 meters tall and 10 meters across. Scientists think many animals digging for many generations made it. So what creatures might dig a tunnel year after year for their burro? There are two possibilities and both creatures may be responsible for different kinds of paleoburros. The first possibility is the lustadon. The lustadon was a huge ground sloth. These sloths were different from the slow-moving tree climbers we know today. Picture a sloth that looks like a hamster but with the size of an elephant. The lustadon was 2 meters tall and 4 and a half meters long. It had huge powerful claws on its hands that it used for digging plants. The lustadon probably dug the taller, wider burros. Another possible animal is the ancient armadillo. Like today's armadillos, they had protective shells on their backs and heads. But unlike today's armadillos, they could grow unbelievably large. The biggest of these armadillos is called the doedicarus. The doedicarus was a meter and a half tall and 3 and a half meters long. It may have weighed 1,500 kilograms. It also had a large spiked club on the end of its tail. It was the size of a small car. Scientists believe these animals built the shorter burros, but they may have made longer, more complex ones also. One great mystery about paleoburros is where these are found. Giant ground sloths and ancient armadillos lived throughout North and South America. But scientists have only found paleoburros in South America. Most are in Brazil, in the states of Santa Catarina and Rio Grande do Sul. These findings may be because most scientists studying paleoburros live in Brazil. But these scientists also search in other places. But Brazil seems to have the most of these prehistoric burros. We just do not know why. Greg McDonald is a scientist who studies ancient ground sloths. He is from North America. He spoke to Discover magazine about North America's lack of paleoburros. The fact that we do not have them here could be that we have not found them. Or it may be that we had them up here, but we did not have the right kind of soil that let them survive for a long time. Another mysterious thing about paleoburros is the size. Huge animals made these tunnels. But most scientists believe the tunnel's size is greater than necessary. Scientists use the example of the giant armadillo. The giant armadillo is the largest kind of armadillo alive today. They can grow up to a meter long. Large ones can weigh almost 50 kilograms. Like ancient armadillos, today's giant armadillos make tunnels. These modern tunnels are usually 40 centimeters around. Modern armadillo tunnels can be about four and a half meters long. But paleoburros have been found more than 100 meters long. Of course, ancient animals that dug these tunnels were bigger than a modern armadillo. But these paleoburros are larger than a single ancient armadillo or sloth would need. Why were many of these paleoburros so large? The creatures that built paleoburros may have had very complex behaviors. They may have worked together to create huge structures. Many have used these burros to shelter from the cold. Or they may have hidden there from other animals. Scientists today are imagining the lives of these ancient animals. Regrettably, paleoburros are in danger. The study of these ancient tunnels is new. The money for research is limited. Few scientists have the chance to study them. Paleoburros are also in danger from modern technology. Often, people find these during building projects. By the time they know they are digging into a paleoburro, parts are already destroyed. People build highways and houses on these ancient burros, making it difficult to study what is below. Still, scientists hope more people will soon understand how important paleoburros are. They are trying to raise understanding. One day, we may be able to learn more about the unbelievable creatures that built these burros. Do you remember Dr. Heinrich Frank from the beginning of the program? Since his first discovery, he has become an expert in paleoburros. He and his team have found almost 2,000 of paleoburros. And he supports the idea of studying these mysterious structures. He spoke to the world about why. These are so important. You can remake the home of a prehistoric animal. This research does not exist in any kind of ancient animal studies. You have the dinosaurs. You have their bones. But you do not know where they lived. But we find the homes of ancient animals. Our group is working in a new area. Do you like ancient animals? What is your favourite dinosaur or ancient mammal? Why? We want to hear your thoughts. You can leave a comment on our website or email us at radio at radioenglish.net. You can also comment on Facebook at facebook.com slash spotlight radio. The writer of this program was Dan Christman. The producer was Michio Osaki. The voices you heard were from the United Kingdom and the United States. All quotes were adapted for this program and voiced by Spotlight. You can listen to this program again and read it on the internet at www.spotlightenglish.com. This program is called Brazil's Most Mysterious Tunnels. We hope you can join us again for the next Spotlight program. Goodbye.