Most unique hostel you've ever stayed in?

Hi everyone, I love travelling for the sole purpose of staying in really cool hostels, so I'm wondering if anyone has any recommendations for an incredibly unique hostel. This doesn't need to be the best hostel experience you've ever had, just the most unique and cool. This can be the actual setting/architecture of the hostel or a unique activity/excursion that the hostel offers.

Some examples of unique hostels I've stayed in:

1. Treehouse Hostel (Granada, Nicaragua) - as the name says, a massive treehouse turned into a hostel

2. Viajero (Lima, Peru) - hostel located in a 112-year old mansion, incredibly stunning common areas with tons of period features