If you go to Gisenyi you can enjoy great views of Lake Kivu, one of the Rift Valley lakes in Africa, and walk along the shore. The border of Rwanda and Democratic Republic of the Congo runs through the lake from north to south. Its largest island is Idjwi. With a surface elevation of 1.462m, a length of 89 km and a width of 48 km respectively, it belongs to the larger lakes of Africa, as ranks its depth (some 485 m) it among the twenty deepest lakes of the world. But what makes Lake Kivu unique is its huge content of dissolved methane (CH4) and carbon dioxide (CO2).
It is estimated that currently approximately 65 cubic kilometers of methane gas is solvated in the deeper water layers of the lake as well as 265 cubic kilometers of carbon dioxide gas. The figures are permanently increasing, and an extrapolation results in the expectation that at the end of the 21st century the water layer will be maximally saturated. This is of serious interest as this situation could cause a sudden gas burst with fatal consequences for the roughly 2 million people living along the shores. The carbon dioxide originates from volcanic active areas under the lake. Bacteria transform parts of the carbon dioxide into methane gas. Both types of gases are dissolved in the deep water. You may wonder why especially methane doesn't release into the air, since it is an unpolar gas with very low solubility in polar water. Only very few lakes are able to store gases to an substantial amount like Lake Kivu does. It requires three features: a warm water temperature, a considerable depth, and a source of gases at the lake's ground. Due to the enourmous pressure of the water layers above the carbon dioxide and the subsequently produced methane can't release but is stored - like the gas in a bottle of champagne. The system is currently stable since the saturation concentration is even higher. But there exists a danger that a distortion arising from limnic eruptions could cause a sudden release of the gases – comparable to opening a bottle of champagne.
The fragile situation is thought to be overcome by controlled degassing of the lake. The principle idea is to install a vertical pipe into the deep layers and pump the gas rich water close to the surface. Due to the lower pressure the gas will start to bubble. Fortunately enough, one is additionally able to separate the methane gas from the carbon dioxide since the chemical different species start bubbling at different pressure conditions. This allows to harvest the methane gas and to make use of it. From the Earth caches location you can currently see a platform positioned some hundred meters in the lake being part of a pilot plant for electricity production. While this approach seem to combine a controlled and necessary degassing with valuable energy production, some people fear that the pipes extracting the deep, gas rich water could already mean a serious distortion with unforeseen consequences. Let’s hope that the engineers will do a proper job.
To log the cache, you should visit the location and find an information board. Please send an email to the owner answering the following questions:
1. What is the age of the lake according to the given information?
2. Below that level are the gases accumulated?
3. The information board gives an explanation of what can happen with the gases. Please describe it and give a reason why it doesn't currently occur.
4. If you like post a picture of you at the nearby shore.
Update 2016-10-22:
It appears the information board in Gisenyi isn't there anymore. Unless there will be a new one installed, please answer the questions 1-2 by web information and question 3 by explaining your own theory. Alternatively, you can find existing information boards elsewhere around the lake, for instance in Kibuye WP01 (Thanks to hegenor for this information)