Tirss/ Tiris

 plain, invisible

desert plain
Tirss = black earth/ dark ground (Thanks to hadidi71)


Erroneous state attribution by Google

the name of this desert plain you find also in the Mauretanian name of the area Tiris al-Gharbiyya as you can see it on the drawing.
en.wikipedia.org/wiki/Tiris_al-Gharbiyya
It is the southern third of the former Spanish Sahara from ad-Dāchla in the north up to the Mauritanian borders. All along the coast and round Tichla near the southern border it was under Mauritanian control between 1975 and 1979. There was a precise map of the occupied territory in internet. Whoever finds may post it here. Thanks a lot.

Mauritania's northernmost province (in its internationally recognized territory) is similarly called Tiris Zemmur, where "Zemmur" refers to Guelta Zemmur
en.wikipedia.org/wiki/Guelta_Zemmur