PLEASE, for your safety, DO NOT ATTEMPT TO ENTER CAVES. Yanchep's caves are fragile and can easily collapse, injuring you and damaging the caves.
Koala Cave and Tuart Cave,
are just two of over 500 caves in Yanchep. They were formed over 250,000 years ago by underwater streams from water travelling west from the Gnangara Water Mound to the ocean. The water passed through Tamala Limestone which dissolved and formed caves. Due to safety and conservation reasons, most caves in Yanchep do not have public location information, although with research, some can be found.
This geocache sits in between two of the nine entrances to Tuart Cave, named after a tuart tree that grows out of one of the entrances.
More notably, however, is Koala Cave, just 100 meters away at -31.5981, 115.711333 because of its scientific significance.
In 1971, Mr. S.B. Bennett showed Mr, P.J. Bridge, Mr. and Mrs. B.G. Muir, Mrs. S.J. Hallam and a few others a series of caves he had discovered in "Coastal Limestone" near Yanchep, about 48 km north of Perth. One of these, now known as Koala Cave (YN-118), drew attention when Mr. Bridge discovered a dentary bonde of Sthenurus brownei (extinct kangaroo-like megafauna) in a deposit of loose reddish sand within the cave. Later, the group returned to the cave to obtain a small sample of the fauna for preliminary analysis. Dr. D, Merrrlees and Messrs. Bennett and A. Baynes returned on two further occasions to recover more of the material, when Koala bones were also disovered in the cave, and are one of the only known koala bones/fossils in Western Austalia. The entire collection has been lodged in the Western Australian Museum. Koala Cave is a curved fissure trending approximately northeast by southwest. It is probable that the cave resulted from a collapse producing an inclined fissure of the sort discussed by Lex Bastian in 1964, when collapse or subsidence occurs over a weak area, such as an underground cavity, lateral fissures may form. From the shape of the ceiling and floor, it may be concluded that the bottom of this fissure collapsed some time ago.
The cache is not inside any cave or sinkhole - it's in a pyramid of small limestone rocks. The formation is obvious to geocacher eyes.
You are looking for a red keychain micro container.