These are "solution hollows" which have formed on part of a [limestone pavement](http://www.bioref.lastdragon.org/habitats/LimestonePavement.html) in the Brecon Beacons, Wales. These hollows are about 15cm / 6" across and are partially filled with rainwater.
The model was created in Alicevision Meshroom from 162 original images.