Sentence:  Explorers from Bruce to Speke, Thomson and Grant, nought to penetrate its secrets, but the malarial climate, the <WSD>fever swamps</WSD> and tangled forests, not to speak of wild beasts and savage men, barred the way. 
Meaning of the word marked with <WSD> tags:  An area of stagnant water and hot temperatures that acts as a breeding ground for disease vectors such as mosquitos.