Underdown is a 1-hectare (2.5-acre) nature reserve in Farnham in Surrey.

It is owned and managed by the Surrey Wildlife Trust.

[1] This reserve was donated to the Trust by Mrs Underdown in memory of her husband in 1987.

It is a steeply sloping area of gorse and heather together with mixed woodland.

[1] There is access by a footpath from Sandrock Hill Road.