Pits (also known as Ravines or Abysses) are a special sort of trap. A pit itself can be harmless but if it is too deep or filled with hazards such as Lava or Spikes it becomes a danger to Lara's health. It is not uncommon within the Tomb Raider Games and can be found almost anywhere.
In Tomb Raider Underworld there is a trap in which, if Lara falls into it, she will constantly fall as this seems to be a bottomless pit. There is also, strangely, a bright white light at the bottom of it, instead of darkness, showing that is is not an ordinary pit. Some people prefer to call it 'The Infinity'.
If Lara falls into the abyss, she will never reach the bottom. Instead, she dies around 2-5 seconds after she has fallen, then the screen blacks out and you return to your last checkpoint.