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When the hydraulic pressure is removed from the well, small grains of hydraulic fracturing proppants (either sand or aluminium oxide) hold the fractures open. Hydraulic fracturing began as an experiment in 1947, and the first commercially successful application followed in 1950.
Nevada first used the procedure in 1924, and it remained popular for over 50 years, though it has since fallen out of favor due to some unexpected complications (see below). The last gas chamber execution took place in 1999, and only four states still allow it as an option.