HAVANA  - Former Cuban leader Fidel Castro said on Thursday the world was on an "inexorable march toward the abyss," which he blamed in part on the discovery and exploitation of vast reserves of so-called "shale gas" around the world. Shale gas is natural gas locked in rock formations that in the past decade has been found in great abundance around the world and is now considered a top source of future energy. Castro, 85, wrote in one of his occasional columns that "numerous dangers threaten us, but two of them - nuclear war and climate change - are decisive."