What is happening to the iNaturalist website?

What happened to the iNaturalist website?
Since today in the morning the following message appears:

iNaturalist is Catching Some Zs

We’re currently down for unscheduled maintenance.

iNat will be back on its feet/wings/fins/tentacles/pseudopodia as soon as possible.

See https://forum.inaturalist.org/t/inaturalist-is-down/34889/33

Closing this topic to keep discussion of the downtime concentrated to a single thread.