I found this interesting article (in French) from a famous online newspaper (lemonde.fr) about a high mountains range, called “Tumuc-Humac”, which was supposed to be found between south of the French Guiana and north of Brazil. The article explains that the high mountains do not exist. It’s a myth, beginning in 1758 when a Spanish expedition mentionned a mountain called “Tumunucuraque” for the first time… in the today’s Venezuela ! But any way, many other expedition went to this region, between French Guiana and Brazil, searching the high mountains, unknown by the local tribes, but possibly to find the famous Eldorado city. This region was a blank map for a long time, being difficult to access (no navigable rivers). At the end of the 19th c., two French explorers separatly validated the mountains range but we know today that their reports were over-exaggerating the high mountains which are in reality only small hills or “inselbergs” (or “monadnock”) (btw, one of the explorer was killed by a local tribe, so the mapping party wasn’t easy anyway). But the myth of the “Tumuc-Humac” mountains has continued and the name of the mountains range can be found in many maps. And today, Brazil has named a national park “Tumucumaque”. But in the last half century, historians and geographers shattered the myth of a high mountains range and their discovery story.
And what about OSM ? When we search the name “Tumuc-Humac”, we find one street in the French Guiana capital. No mountains between French Guiana and Brazil in OSM. Not a surprise since the area is almost blank, the region is still very wild and difficult to access, a funny location for a mapping party.
But OSM “search” returns also geonames with one “Tumuc-Humac Mountains” ! Surprisingly, wikipedia provides an article about the Tumuk Humak Mountains but doesn’t say anything about the story and the myth about the high mountains range :

“The Tumuk Humak Mountains (Dutch: Toemoek-Hoemakgebergte, Portuguese: Serra do Tumucumaque, French: Monts Tumuc Humac) are a mountain range in South America, stretching about 120 kilometers (75 mi) east–west in the border area between Brazil in the south and Suriname and French Guiana in the north. In the language of the Apalam and Wayana peoples, Tumucumaque means “the mountain rock symbolizing the struggle between the shaman and the spirits”.[1] The range is very remote and almost inaccessible.”

True, it is so inaccessible that even after 50 years of shattering the myth, the name is still official and designates today an area of small hills (500 to 600 meters), far away from the original story of high mountains range possibly hiding a mysterious & ancient civilization.