Why can’t you build a tunnel in swampy soil?

Different types of soil have different density and strength. And therefore, when construction work is carried out, first of all, the characteristics of the soil are taken into account. Swampy lands have a lot of water, they are amorphous and not dense. If a tunnel is made in them without additional fortifications, then the water will begin to seep into the resulting void, and with it huge masses of soil will flood the space.
Therefore, during construction work, swamps are either dried out, or tunnels are laid very deeply, where water does not seep.