But most of them don't and are! That's basically the definition. I think there are three reasonable things to do with a grotto: 1) Add natural substrate and rehouse a smaller species. 2) Leave it as it is for historical purposes and use it either as an isolation enclosure or in some other way integrate it into a modern large carnivore exhibit. 3) As gentlelemur once said: " Swing that wrecking ball!"
or 4. Use them as the basis for a complete renovation/re-invention. If the holding is still suitable this saves a great deal of money. One example of where this has been done: Woodland Park Zoo Blog: New tiger and sloth bear exhibit designs revealed
A slight variation of this method can be found in NaturZoo Rheine. The zoo has an excellent sloth bear exhibit but the old bear grotto is still mostly intact. The zoo makes it very clear that it was once used for bears and has some education around it explaining the progress in husbandry the zoo has made. Guests can enter and use it as a picnic area.
