It could be the drilling machine which people use in traveling, or could be special organisms which are raised as a food source (for example, earthworms), or could be weapons containing unprecedented technologies. Bullet train: Planes were no longer available since the atmosphere changed a lot. However, an effective, high-speed vehicle was needed. People built undergrounds for excursions, but for a journey crossing thousands of kilometers, they would use bullet trains. Bullet trains are much faster underground, with a maximum speed of 1500 km/h. The train would run in a tube which was vacuumed, so that fraction was removed. Therefore, the train was highly energy-efficient and convenient. It only takes seven or eight hours for people to travel from Beijing to Los Angeles. Rig: There were always adventurous people who wanted to visit places where there were no undergrounds. The rig was a vehicle that combined a car with a drill. It was used to make unprecedented tunnels everywhere. The rig was widely used in different scenarios: whether for adventure, mining or opening up new places to live. Though it was convenient, there were several limitations: Rigs were not allowed to be used in cities or villages, since it may lead to collapse which may ruin the whole city. People had to ask for permission if they’d like to use it. What’s more, since there was no fossil fuels available on earth, the rig was powered by electricity, which means they have to be recharged frequently. Mice and earthworms: These are the primary source of protein for people. People could not get access to meat or milk easily since most mammals went extinct as time passed. However, rodents and earthworms managed to survive under the harsh environment since they live underground and they feed on the nutrients from soil and the roots of the plants. They thrived as their predators went extinct. People began to raise those animals in special areas in large cities, and those animals were killed and manufactured into different kinds of protein. Poor people could not afford those meat, so they chose to directly intake amino acids produced by factories. Special-designed guns Rice: Rice was one of the only available sources of starch on the planet since they could withstand a relatively high level of moisture and temperature. In fact, rice grew really well under the harsh conditions since it kept most of their competitors and predators - weeds, birds, etc. away from them.