Pope built the grotto to reach his garden from his villa without having to cross the road separating them. He wanted it to appear completely natural and was delighted when it was completed in 1725. He later developed an interest in mining and in 1739 transformed it with crystals, marble and different types of stones. The grotto is open to the public during the Twickenham Festival and certain other dates.
https://popesgrotto.org.uk/