One of the most interesting places ①were Manjanggul which ②is the longest lava tunnel in the world. Outside the cave the temperature was more than 30 degrees, but just a short distance into the cave the temperature suddenly dropped to 18 degrees. It was ③like entering a huge refrigerator. Inside the cave I tried to imagine ④that this tunnel ⑤cannot have been like about 25,000 years ago. At that time a river of lava rushed through it with more energy than an atomic bomb.