Arma 3: Points of Interest
"Atlantis" can be found at 150068 (source)
Altis also has its (nicer) Stonehenge with a decorative skull on the top, located in the North-East corner of 144061 at Cape Makrinos.
A hidden hideout (and what is supposedly its owner's skeleton) can be found at 080094 (source)
The mentioned Pirate Cave is located at 042073 (how-to video)
V-22 Osprey wreck
The Bushlurker Monument
At 027074 lays a monument in tribute to Paul “Bushlurker” Pelosi (1960-2017) who has worked on many community terrains, including Malden for CWR2. He passed away some months before Malden 2035's release.