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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?

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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

enter image description here

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

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  • $\begingroup$ "Soil horizon" was what I was looking for. Thank you! $\endgroup$ – Martin Thoma Sep 23 '15 at 18:22
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First of all we must consider what the substrate is at 30 meters and deeper, either clay, anaerobic, impermeable or permeable substrates, everywhere that you find liquid water between 0-100 degrees you find life (Geogemma barossii reproduces at 120'C 250'F), even if the peak of everest is mostly under zero and has 270kph winds, algae grows there.

Plant life requires light, so you will only find roots underground, the deepest roots are an african fig that has a record of 120m 400ft. Most plants have roots that can go as deep as the tree is high in permeable ground while clay soil is very low on oxygen and voids for anything to infiltrate. Underground mining will rarely affect tree roots, or any other notable biotope, unless it encounters a rare cave system with fish and so on.

In very deep caves 200 meters deep there can be all kinds of insects and fish, although that doesnt qualify as ground.Krubera cave georgia has insects at 1.5 kilometers depth.

The solution of the question is that of matabolism, anaerobic versus aerobic, and it's effects on biotope richness. Funghi breathe and bacteria can be anaerobic, so they are the deepest, you can find some in oil wells and 1.6km under the sea floor.

The major concerns for drilling and damaging deep biotopes are from mining isolated aquifers with endemic animals, for example the deep lakes of antarctica that have been set apart from the rest of the world for 10's and 100's of thousands of years.

The ecosystem richness of zones without light and oxygen and flowing water is generally too low to foster much attention from any environmentalists and conservation organisations, it's also not a rare biotope.

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