In July of 1999 I teamed up with Christoph Weber of Germany and his clients, a German "volcano chaser" (Wolfgang Mueller) and a film crew who were making a documentary for German television. Celia Nyamweru from St. Lawrence University also joined us for 3 nights in the crater. (Celia is a specialist on the Rift Valley who has been studying Ol Doinyo Lengai since around 1988.) The German team camped in the crater for 9 nights, and I camped with Paulo, my guide, for 15 nights (July 23-Aug 7).
During the visit, nearly all activity occurred in spatter cone T40. For the first 9 days it contained a lava lake that became progressively larger and more active. On the night of July 31 the lava lake filled T40 nearly to overflowing, and powerful degassing created holes in its weak northern rim. Lava escaped and flowed northeast onto the floor of the main crater. The most interesting activity occurred from Aug 4-5 when a small explosion blew open a vent on the flank of T40 and spectacular lava splashes formed a new spatter cone. At the moment that eruption began, I was lying in my sleeping bag only about 15m away!
For a few days in early August, T37N1 also contained a lava lake at a depth of around 20m.
For more details, read the Smithonian Global Volcanism Network Reports which can be accessed from my links page.