https://en.wikipedia.org/wiki/Hermitage_cats
"The cats have been present in the museum, originally a palace, since the 18th century;[1] in 1745, Elizabeth of Russia ordered cats to be placed in the palace in order to control the mice"
Russian version tells that there are almost 20 km of basement tunnels/hallways there https://ru.wikipedia.org/wiki/Эрмитажные_коты
https://en.wikipedia.org/wiki/Hermitage_cats
"The cats have been present in the museum, originally a palace, since the 18th century;[1] in 1745, Elizabeth of Russia ordered cats to be placed in the palace in order to control the mice"
Russian version tells that there are almost 20 km of basement tunnels/hallways there https://ru.wikipedia.org/wiki/Эрмитажные_коты