He had already logged more than 50 free dives at 100m+ (328ft+) and was
apparently training for an attempt to beat the (unofficial) record (I think
it stands now at 123m/403ft) due next June.
These "no-limits" free dives are basically variable-weight dives, where the
apneist descends pulled by a sliding weight, and ascends by inflating some
air balloon. On march 29th, Isoardi's coach, waiting on the boat, saw the
inflated balloon and weight coming up alone. The attached depth gauge read
124m (406ft). Cyril never surfaced. What really happened is unknown since
there were apparently no assistance SCUBA divers at depth.
In a press release, the French diving Federation (FFESSM) expressed regrets
for the accident but insisted that both FFESSM and CMAS do not recognize
any free diving records.