Limestone

Can somebody who knows geology write a concise entry here? That should do for now - in the future, a full entry might be in order, or

- what is limestone?

- why/how/when was it created?

- why are so many caves in limestone?

- etc

Bear in mind there's probably no point in duplicating information if the wikipedia entry is accurate and does the job: http://en.wikipedia.org/wiki/Limestone