a, Original Boehler–Almax-type single-bevelled diamond anvils with 40-μm culets (before milling). b, Greyscale bitmap used for milling. c, The milling depth profile. d, Crafted toroidal culet. e, Cross-sectional schematic of the dsDAC assembly (not to scale): a gasket (grey) with the pressure chamber (dark grey; the sample is yellow) squeezed between the two toroidal diamond anvils (light blue) equipped with hemispheres of nanocrystalline diamond (white); the pressure chamber is of 6 μm in diameter and 4 μm in height. The gasket was prepared as follows: We used strips of a 200-μm thick Re foil. To make an indentation with a thickness of about 4 μm, we followed a many-step procedure. First, the Re foil was pre-indented to a thickness of ~20 μm using a pair of single-bevelled diamonds with 80-μm culets. Then, a hole about 30 μm in diameter was laser-drilled in the centre of the indentation, and the indentation was pressurized again between the same bevelled diamonds. This led to closing of the hole and a reduction in the thickness of the indentation. This procedure was repeated a few times until a thickness of about 7 μm was achieved. The gasket was mounted into a BX-90 DAC equipped with toroidal diamond anvil and indented by their miniature 10-μm culets to a thickness of ~4 μm. A hole (~6 μm in diameter) in the centre of the ~10-μm indentation was made using FIB or by tightly focused pulsed near-infrared laser to form a pressure chamber.