This is a great article posted on another site (who reposted it from another site, who took it from another), but I thought many would enjoy it here. It is The Lazy Man’s Guide for Constructing a Call of the Cthulhu Adventure, written by Sandy Petersen, original author of the Call of Cthulhu.
The runoff from Mount "Evil Volcano" has turned an area of small lakes below into a vision of hell. The mixture of acids in the lakes is of such strength that virtually any organic material touching is quickly dissolved. It is a melange of sulfuric, nitric and hydrochloric acids.
However, the place does harbor life, and its very strange, and very dangerous....