[1] The deep waters of the East African Rift Lake Kivu contain large amounts of dissolved carbon dioxide and methane. The release of a fraction of these gases, which could be triggered by a magma eruption within the lake, would have catastrophic consequences for the two million people living on its shore. Up to now the safety assessment of the lake was based on the assumption that the gas concentrations in the deep waters are in a steady state with a residence time of 400 years. Turbulent transport was regarded as the main pathway of vertical exchange. Recent measurements and the analysis of the vertical transport processes in the lake radically change this evaluation. The vertical turbulent exchange is negligible, as documented by a spectacular set of several hundred double-diffusive layers. Gases are mainly transported out of the deep zones by a slow upwelling with a residence time of 800-1000 years. Our results indicate that the methane production within the sediment has recently increased, leading to a gas accumulation in the deep waters and consequently decreasing the heat input needed to trigger a devastating gas release. With the estimated current CH 4 production, the gas concentrations could approach saturation within this century.

During the eruption of Nyiragongo Volcano in January 2002 about 10 6 m 3 of lava entered Lake Kivu. The high concentrations of CO 2 and CH 4 dissolved in the deep waters of Lake Kivu raised serious concerns about a potential gas outburst with catastrophic consequences for the population in the Kivu-Tanganyika region. Therefore, 3 weeks after the volcanic eruption, we performed an ad hoc lake survey of the stability of the water column stratification. Vertical profiles of temperature and turbidity revealed signatures of the lava, which had penetrated to 100 m depth; however, there was no substantial warming or destratification of the gas-containing deep layers below. The deep double-diffusive structures also remained unaltered. Based on these observations, we conclude that a thermally driven gas outburst in Lake Kivu is not to be expected from future eruptions of comparable dimensions. In addition, the recent measurements allowed for an update and gave new insight into the stratification and double-diffusive mixing phenomena in Lake Kivu. A comparison with former measurements revealed a warming of the upper part of the lake of up to 0.5ЊC within the last 30 yr, which could be attributed to climate variability.

The soil‐gas 222Rn concentration had been monitored almost continuously from June 1993 till November 1996 on Taal volcano, Luzon Island, the Philippines. During this measurement period, a singular Mb 7.1 earthquake occurred on November 15, 1994, between Luzon and Mindoro, 48 km south of the volcano. Twenty‐two days before the earthquake, an anomalous increase in soil‐gas radon (peak to background ratio = 6) was recorded, unique in the whole time series. The possible generation of this anomaly by typhoon Teresa, which struck Luzon Island a few days before, was ruled out one year later when super typhoon Angela, the most powerful storm to hit the Philippines in ten years, crossed Luzon Island along almost the same track without triggering a similar disturbance in the radon signal. Consequently, there is strong evidence that the Taal radon anomaly originated in stress accumulation preceding the Mindoro earthquake.