It is absolutely dangerous to dig tunnels under the enemy's noses, and it is not enough to dig them in a day or two. The possibility of exposure is very high, but Luo Zheng has no better way, so he can only pray for darkness under the lamp and get away with it. However, Luo Zheng did not put all his hopes on the method of digging tunnels. After agreeing to Cao Xi's proposal, he looked around and whispered: "We wait until dark here, and it is easy to be exposed when we run back."

"Okay, it's not long since it's dark." Lan Xue said in a deep voice, glanced at Cao Xi, and asked with a serious face: "Although the place you chose is somewhat concealed, it is less than two thousand meters away from the base. There is a problem. The Dark Prison is underground. Do you know where? Don't dig it off."

"This is not the case. The prison should be somewhere outside the base. I still need to observe for a while before I can finally make sure. Give me some time." Cao Xi whispered, with a strong sense of confidence on his face.

"Why is it the periphery, not the base? The entrance to the Dark Prison is located inside the base and is not better protected?" Lan Xue asked curiously.

"This is not contradictory." Cao Xi whispered, and saw Lan Xue's curious expression on her face, she explained: "The entrance must be in the center of the base, and it is difficult to get over it, but the Dark Prison cannot be under the base. You think, how can the water for nearly 20,000 people be discharged? It will definitely penetrate into the ground. Over time, it will penetrate below two hundred meters, causing the Dark Prison to rust, collapse, leakage, etc. This is not advisable. Therefore, the Dark Prison must be on the periphery, while the passage is inside the base."

"It makes sense. If this is the case, the Dark Prison should be a distance from the underground river and is in the upper reaches. Otherwise, the daily water in the Dark Prison cannot be discharged. In other words, there must be an underground river below. We only need to find the location of the well to determine the approximate location of the upstream of the river." Luo Zheng said in contemplation.

"You are right. Whether on the ground or underground, domestic water must be discharged. It is impossible to dig a drainage channel here and let domestic water pour in randomly. Over time, it will change the geological structure of the base. You can only use underground rivers to drain water. The domestic water on the ground leads to the ground through drainage wells. The wells for food are in the upstream, and the wells for drainage water are connected to the downstream. Therefore, we only need to find the locations of the domestic water wells and sewage wells." Cao Xi praised.

"It makes sense. It seems that there is a great possibility that there is an underground river below. The Dark Prison is in the upper reaches of the Underground River. In this way, as long as the direction of the Underground River is found, the location of the Dark Prison can be roughly judged." Lan Xue said in surprise.

"The Dark Prison has a drainage channel connecting the underground river, which may allow us to penetrate." Luo Zheng looked at Cao Xi in surprise and asked. Seeing Cao Xi nodding affirmatively, there was a little more information, and continued to ask: "Does it mean that no matter where we dig tunnels, as long as we lead to the underground river, follow the river, you can find the drainage channel, and then penetrate into the Dark Prison?"

"There is a problem. Everyone has a problem with swimming, so I'm afraid there is no air in the underground river." Lan Xue reminded.

"No, the underground rivers under the desert will not be very large, but the soil is relatively loose, the river channel will not be small, the space is large, and it is also full of living water. If there is air, it is cold." Cao Xi denied.

"If you can't be afraid, our equipment has waterproofing capabilities, the most urgent thing is to dig the underground river." Luo Zheng said indifferently, quickly checking the base, eager to find the direction of the underground river.

"I have roughly been able to determine the location of the underground river. The tall hill I just mentioned is the lower reaches of the underground river. The distance is about 200 meters deep. Have you noticed that the overall terrain will sink further. If we can find a lower terrain, then the time we dig can be much shorter." Cao Xi said affirmatively.

"Can you use aerial coordinate maps?" Luo Zheng asked in surprise.

"Okay, let me see." Cao Xi agreed.

Luo Zheng quickly took out his phone and called out the three-dimensional topographic map of the base. It clearly indicated where there were sand dunes, where there were sand valleys, and how many altitudes were. Luo Zheng looked carefully and found that the altitude had indeed begun to decline to the side. He couldn't help but be overjoyed and quickly handed the phone to Cao Xi.

Cao Xi took it over and carefully distinguished it, without missing any details. While observing the actual terrain around, Cao Xi pointed to the direction of the tall sand dunes in front with a certain expression and said, "Walking there, there is a valley about five kilometers, and the sand valley is covered with sea buckthorn. Less than a hundred meters below should be an underground river."

"Five kilometers?" Luo Zheng looked at Cao Xi worriedly.

"Don't worry, the underground river is living water, the cave should be quite large and has air." Cao Xi said confidently.

"Okay, let's go to the place you mentioned. It's still too close and it's too deep from the underground river, so it takes time to dig it." Luo Zheng said in contemplation, looking up at the direction Cao Xi was talking.

"Go now." Cao Xi suggested.

Luo Zheng thought for a while, looked at Lan Xue, and Lan Xue nodded knowingly and said, "Yes."

The three of them got up, first looked around with vigilance, and then quickly trotted towards their destination without finding any danger. Five kilometers were not far away. However, in order to avoid reconnaissance, the three of them circled the hills as much as possible and walked a little more road. In addition, they encountered a patrol team on the road, which delayed them by more than ten minutes.

An hour later, the three of them came to the valley mentioned by Cao Xi. Sure enough, they saw that the valley was covered with sea buckthorn, which was more than 1.5 meters high, and about half a meter short, and full of fruits. Luo Zheng said in surprise: "Even if there is no access to the underground river here, it is worth it. So many sea buckthorn fruits are just enough to replenish everyone's water and vitamins."

"The aerial photos you showed me only saw sea buckthorn trees. I didn't expect that the sea buckthorn trees here are so high. It's so good. It has a lot to do with the underground river below. I judged that the underground river is no more than 30 meters away from the ground. I can dig a passage in two hours at this depth." Cao Xi said in surprise.

"Two hours?" Luo Zheng looked at Cao Xi in surprise. Thirty meters was not shallow. It took only one night to dig with all his strength. Cao Xi actually said that he could dig in two hours. He asked in surprise: "You can't joke on the battlefield. Are you sure you can do it in two hours?"

"No problem, don't worry." Cao Xi said confidently.

"The manual digging speed is not fast. Do you have any solutions?" Lan Xue was curious about the temperature.

"Of course, you will know in a while." Cao Xi said confidently.

"Okay, it depends on you." Luo Zheng looked at the sky that was almost dusk, and the sun was hanging in the west, red and sunset was dense, dyeing the west sky red and the temperature began to drop. He couldn't help but say in a deep voice: "I will contact the large army immediately and ask them to come here to meet, and the team will take action as soon as the team arrives."

"No need to wait, I'm starting work now." Cao Xi said confidently.r1148

...
Chapter completed!