The 'ice' cave, as its Bulgarian name translates to, is 16km west of Vratsa, within the Vrachanski Balkan Nature Park. It's sheathed in ice for much of the winter, but thaws in summer, when visitors on guided tours arrive to explore it. There's no public transport, so you'll have to hike (three hours from Vratsa) or take a taxi (15 lv).
Wear shoes suitable for climbing, and dress warmly, even on a hot summer day.