I hear almost about the speed of light and faster to return to past. I believe the speed of light belongs to quantum level of matter or a supergravity space for macro matter. For atoms, the density over a coordinate of y= density and x= atomic mass of elements is like a “ro” ρ in italic Greek alphabet on the planets of solar system. For the sun and galaxies, it is a bigger “ro ” ρ and so on. On a quantum level the sub particles can be in several places, behaving exactly the same, it means, the mater has several similar layer for sub particles state in different places!? It is what the theory of M says. Some sub particles are present everywhere. To speed, you need some accelerator and applying energy. Or to be in a neutron star that could have his own gravity rules. We know for a sub particle we can have the place or the speed of this sub particle in a precise time. We cannot predict both in the same time. We know the state of the matter is changing and if something speeds more than speed of light, it can be found somewhere else. You never come back to the same past. It is very important we know the basics.
Another subject is the sinking of the soil. I hope I use the word correctly. Like the water washing some layer of the soil under some layers, and the upper layer comes down or slides. It is sometime, the case for metro tunneling. As the metro channel is going through, some upper layer fall down. The support of the soil deep under is not the same and forces does not behave as before. You have sinking of the soil without the metro construction as well. You need the schlumberger electrical array of electrodes and soil resistance technics to predict what you are facing some meters before you go through. Or if you can develop a network of elements made from electric electrodes implanted in a network under the surface to see what is changing in your city soil.