I was looking at the Coa Valley-site, and noticed that it was only discovered in the late 1980s. Two more sites that were discovered since, let's say, 1970?

Part of the "Land of Frankincense" site i.e. the Archaeological site of Shisr (thought to be the historic site of Ubar) was only discovered in/around 1992 using satellite photos as a guide. Not the entire site but would meet a "significant element" rule.
for refs see
http://ngm.nationalgeographic.com/ngm/0502/feature1/index.html
("did you know?")
and
http://www.etravelblackboard.com.au/showarticle.asp?id=84937&nav=21


The Mausoleum of the First Qin Emperor was famously discovered by some peasants working in a field in 1974. When we visited Xian in 1978 it wasn't yet open to the public and we had to make do with a single statue in Xian museum - things had changed by the time of our second visit in 1989!!


If you stretch your 1970 rule a bit then I believe that L'Anse aux Meadows archaeological site was only discovered in 1960 by a norwegian archaeologist looking for norse settlements in the "New World".


I note that the Thracian Tomb of Sveshtari is stated in the Advisory body evaluation to have been discovered in 1982. I have been there and it is a huge great mound which couldn't be missed by anyone - including the very nearby local villagers/farmers!!! This Web site calls it a "large hill" ( http://en.journey.bg/bulgaria/bulgaria.php?&city=2218&gtype=10&gsubtype=9&gtype=10 ). This gives entirely the wrong impression regarding its visibility -it is clearly a tumulus -see photo with my review http://www.worldheritagesite.org/sites/sveshtari.html . So it seems surprising/amazing that no excavation at all had happened before and I can find no other evidence for the statement (except other Web sites which just repeat it). I note that the inscription goes back to Bulgaria's "bad old communist days" when truth wasn't necessarily a valued commodity and ICOMOS evaluations were rather less thorough than now. Indeed 3 elapsed years from "discovery" in 1982 to "inscription" in 1985 seems to stretch credulity but I guess we have to follow the UNESCO documentation?

So I think we need to be careful about what is meant by "Discovered". It does appear that the particular significance of the Coa valley petroglyphs only became known to relevant scientists late on when a dam was planned - but presumably many were known to locals as they were outside in the open air. It does appear that specific "fine" carvings have only been discovered as late as the 90s once scientists started looking hard.

You need to bear in mind also the exisitng and similar connection of "Recorded Discoveries" which would overlap this one. It was really intended to be geographic locations rather than detailed archaeological discoveries but currently includes Machu Picchu and Moenjo Daro

The Macedonian Tombs at Aigai/Vergina were discovered only in 1977, whereas the city was discovered as early as 1861.