Coal the heat and pressure of deep burial causes the loss of water, methane and carbon dioxide and an increase in the proportion of carbon. Thus first lignite (also called "brown coal"), then sub-bituminous coal, bituminous coal, and lastly anthracite (also called "hard coal" or "black coal") may be formed. The wide, shallow seas of the Carboniferous Period provided ideal conditions for coal formation, although coal is known from most geological periods. The exception is the coal gap in the Permian–Triassic extinction event, where coal is rare. Coal is known from Precambrian strata, which predate land plants—this coal is presumed to
What If?: Serious Scientific Answers to Absurd Hypothetical Questions the Month". The book was a Goodreads Choice Awards "Best Nonfiction" nominee in 2014 as well. What If?: Serious Scientific Answers to Absurd Hypothetical Questions What If?: Serious Scientific Answers to Absurd Hypothetical Questions is a non-fiction book by Randall Munroe in which the author answers hypothetical science questions sent to him by readers of his webcomic, "xkcd". The book contains a selection of questions and answers originally published on his blog "What If?", along with several new ones. The book is divided into several dozen chapters, most of which are devoted to answering a unique question. "What If?" was