How does life change when you dig deeper? I've just realized that I have no idea what life / biotopes / soil looks like when you dig deeper than a few meters. I know that in the first meter of soil you can find all sorts of live animals (like moles and rabbits), insects (like ants, but many more), plants, and single-celled organisms, of course. But how does that change when you get to the depth of a metro station (below 30m)? When you plan to build a metro, do you have to consider that you might destroy a biotope? I guess there is a point from which you will quite certainly not find animal/plant life in the soil anymore. This will, of course, depend on where exactly you are. But can you give a rough estimate of how deep we're talking? 50m? 100m? 1km?

Extremophile bacteria and archea are living very deep beyond our imaginations. This is what you need for general composition at different depths. And this and this is for the deepest living organism known ! Following is image from (Manson et al 2010) which shows at what depth you will get bacteria

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Even more deeper, **Wold's single species ecosystem** , _Desulforudis audaxviator_ which can be found 3 km below sea level.