The Ruins of Nosti (more commonly referred to as The Church Cave) is a Cave underneath a ruined building on the Scorched Earth map. The ruins above are a forgotten city full of domed buildings and pillars in the south east of the map in the dunes. The largest domed building being the entrance.
The cave that is the Ruins of Nosti begins with a descent down beneath the surface. This location dubbed as the entrance consists of the Church all the way down to where the path splits into two, where it then becomes the descent.
This is easily the most dangerous cave in Scorched Earth between the jumps over a bottomless void and the combination of some of the most dangerous creatures—including large swarms of Onycs and Mantis. Survivors should be well-prepared before tackling this cave. Mega Rabies will come from the bats, so make some Lesser Antidote beforehand. However there is not quite the variety of creatures here that there is in other caves, but don't take this as a sign to underestimate the cave itself.
Beware of the "Trap Loot" below and to the right of the series of jumps there is a loot crate near the instakill area on a ledge however it will kill you before you even reach it.
Bringing a decent Thylacoleo is an option, because it can climb up the ledge to the left of the "Killvolume" area. After that you have direct access to the south part where the artifact is - so you don't have to do the dangerous jumps. Also it has decent weight, so one could farm the Obsidian inside the cave.
For some reason, small creatures will spawn upon entering the cave for the first time. However, "swarm" creatures will spawn AFTER you acquire the artifact and will attempt to stop you leaving with the artifact. Don't be tricked by this mechanism, bring powerful tames with you.
Prior to patch 292.100, the cave entrance requires the survivor to enter the cave through crawling under boarded boards at the old gate, preventing most creature from entering or leaving the gate. This makes raising a decent Thylacoleo behind the the gate entry the only one-way option of ensuring a quick passageway to the cave through climbing up the ledge, bypassing the "Killvolume" area.
According to Raia's Explorer Notes, this city was once the city of Nosti, a large settlement where worship of the Obelisks flourished, until the Obelisks destroyed the city many decades ago.