  We study the development of a negative discharge driven by a Marx generator
of about 1 MV in an air gap of 1 up to 1.5 meter, at standard temperature and
pressure. We show the evolution of the discharge with nanosecond-fast
photography together with the electrical characteristics. The negative
discharge develops through four well-distinguished streamer bursts. The
streamers have different velocities and life times in different bursts. The
last burst triggers a positive inception cloud on the positive grounded
electrode and a burst of positive counter-streamers emerges. The pre-discharge
then bridges the gap and leaders grow from both electrodes. Finally a spark is
formed. Looking closer into the pre-ionized zone near the cathode, we find
isolated dots which are potential branching points. These dots act as starting
points for positive streamers that move towards the high-voltage electrode. We
also find such phenomena as space leaders and leader stepping in our laboratory
sparks.
