In reality, there are two reasons gravity trains do not exist. First, a lengthy transit distance would pierce the Earth's mantle and traverse a region where rock is more fluid than solid. No materials are known that would withstand the tremendous heat and pressure in the inner core. Temperature is estimated as 5,700 K (5,430 °C; 9,800 °F), and pressure as high as about 330 to 360 gigapascals (3,300,000 to 3,600,000 atm). Secondly, frictional losses would be significant. Rolling friction losses could be reduced by using a magnetically levitated train. However, unless all air is evacuated from the tunnel, frictional losses due to air resistance would render the gravity train unusable. Evacuating the atmosphere to make it a vactrain would eliminate this drag but would require additional power. Such objections would not apply for solid planets and moons that do not have an atmosphere.