What's with the monstrous footprints?

[Tenchuu Khan]

I mean - when we can see BTs they are always human shaped floating in the air, but in the cutscenes it always looks like some monster is causing those prints.

[Vahnkiljoy]

They are giant human hand prints, you will get some emails later that sheds some light on the meaning.

[Tilki Andrei]

It happens when they are moving around. If you make too much noise and trigger a BT, they will start coming towards you. It will still float but as it moves, it will leave footprints on the ground.

This is how they interact with the world because of their nature.

Also, normal people can't see the floating ones even with a BB and scanner. Their only way to find out where a BT is to look for footprints and match it with their scanner.

Sam is able to see them only after his senses gets combined with a BB.

Rest is spoilers.

[Tenchuu Khan]

Ye just going for my second playthrough and I'll take more time now reading the mails and what not.
I guess I have missed out a lot of the background stories by not reading the mails and interviews.

[David Sarif]

In some concept arts, BTs actually had extra appendages (hand-shaped) that would connect to the ground.

...but yeah, currently, and canon, is these handprints being their gravitational influence in our plane, even if they're actually floating about.

[SpikyMuffin]

It may be a bad example, but in anime Elfen Lied superhumans have invisible human hands going out of them, that they control and easily kill someone. I think this idea is somehow related to japanese culture and Kojima got it from this or some other source of japanese culture.
PS still didn't read all interviews, probably some explanation there