This is a sister cache to 'Out in a limb' a little further along this Rata St track. Is it a worm hole, a black hole, or just an ephemeral organic dark space that will eventually be reclaimed by the forest?
According to Wikipedia, a black hole is "a region of spacetime exhibiting such strong gravitational effects that nothing—not even particles and electromagnetic radiation such as light—can escape from inside it". However, a worm hole is "a theoretical passage through space-time that could create shortcuts for long journeys across the universe. Wormholes are predicted by the theory of general relativity. But be wary: wormholes bring with them the dangers of sudden collapse, high radiation and dangerous contact with exotic matter."
Don't despair - the safety and comfort of cachers is not threatened by this slightly sneaky hide (no need to leave the track), and you will remain in the current dimension of the space-time continuum.
BYO own pen and log extraction tool as while the cache is small the log is tiny.