Seracs
Seracs (from Swiss French sérac, originally the name of a compact white cheese) are house-size blocks of glacial ice, sometimes with their own 'entrances', 'windows' and 'rooms'. They are formed by intersecting and widening crevasses on the glacier.
Crevasses, in turn, are formed when glacial ice sheets are fractured by horizontal and vertical shearing forces when the glacier glides downstream. When these crevasses become wide enough, they form 'lanes' and 'streets'. Sometimes, snow would bridge over the tops of seracs, turning them into caves or tunnels.
Tasman Glacier, 2015