There are some places, where are very dangerous caves that they are perfectly inaccessible and unfit for habitation. Not to speak of other creatures, even birds can not cross them.
सन्ति क्वचिन्महादर्यो दुर्गाः काश्चिद् दुरासदाः। नातिक्रामेत पक्षी यान् कुत एवेतरे मृगाः॥