This is perhaps the
most important thing to
observe while
caving,
urban exploring, or really
anywhere in
nature.
It is pretty self-explanatory, don't take anything, don't leave anything. At all. Don't steal. Don't litter.
Even a small rock that might seem insignificant, leave it there. If everyone took a rock, or an arrow head, even a mountain would be reduced to rubble.
Even a small mint wrapper that might seem insignificant, take it with you. If everyone left a mint wrapper, even a massive cave would be filled over-night.
There is however, one exception. Feel free to take as much trash with you when you leave. If everyone took out a backpack full of trash, even a garbage dump could be emptied out easily.
Also, keep in mind that sometimes even footprints can be damaging. Make sure to avoid fragile areas and be concious of where you step. (Thanks momomom)
It's really quite simple. These exotic sites need to last for generations to come. Don't ruin them because you're too lazy to carry your beer bottles out with you.