This video says about itself:
18 October 2013
In 1986, the hospital closed down. The area became a nature reserve. Various wildlife species started to use the swimming pool. Prominent among them, blue hawker dragonflies.
There is a report by Sjek Venhuis on blue hawkers at that pool in 2013-2017 on the Internet. In summer, adults of this beautiful insect species come there to mate. Females deposit their eggs on the banks of the old swimming pool. In winter, dragonfly larvae crawl on the bottom.
The old pool is now one of the most important blue hawker spots in the Netherlands. In 2017, at least 200 females deposited eggs there.