Much greenhouse gas emissions comes from uncontrolled underground coal fires. I can't find any detailed source on its share of global CO2 emissions; I see estimates for both 0.3% and 3% quoted for coal seam fires just in China, which is perhaps the world's worst offender. Another rudimentary calculation said 2-3% of global CO2 emissions comes from coal fires. They also seem to have pretty bad local health and economic effects, even compared to coal burning in a power plant (it's totally unfiltered, though it's usually diffuse in rural areas).
There are some methods available now and on the horizon to try and put the fires out, and some have been practiced - see the Wikipedia article. However the continued presence of so many of these fires indicates a major problem to be solved with new techniques and/or funding for the use of existing techniques.
Coal seam fires were not mentioned by Let's Fund or by GWWC. Coal seam fires were not mentioned in the Founders Pledge climate change report, but we can plug it into the methodology in the report. We can estimate that the world's fires will cause 30 gigatons of CO2e emissions by 2050 (based on 3% of current global emissions, and assuming that the absolute amount will be fixed through 2050) which gives 4 points for importance. I can't find any examples of philanthropic funding dedicated for the issue, so it gets 16 points for philanthropic neglectedness.
Meanwhile, the US seems to face a little more than $1bn in expenses for all completed, current and projected coal fire projects, which could mean a few tens or hundreds of millions of dollars per year. This op-ed from last year accuses environmentalists of wholly ignoring the problem of coal seam fires. Jay Inslee's 'Green New Deal' proposal (probably the most ambitious and detailed) does not explicitly mention the issue. So the the government and private sector even in all countries can be presumed to spend less than $4bn on the problem, giving it 16 points for non-philanthropic neglectedness too. This creates a score of 4 + 0.5*(16+16) = 20 points, tying it for 4th place among 7 climate change efforts, though I don't think this methodology is really accurate.
Another approach is to directly estimate the cost-effectiveness. The various early failed and rejected attempts to extinguish the Centralia, PN fire each cost $80,000-$360,000 in today's money according to the Wikipedia article, so we might imagine that an early, quick $500,000 of extra funding would have extinguished it, but this relies on hindsight (knowing that the existing efforts would fail, and knowing that the coal fire would grow so much) so let's assume that a strategy of going the extra mile in extinguishing ten potentially big coal fires (=$5 million) would have saved Centralia. (The US has probably already updated to much higher standards, but India/China/Indonesia may have plenty of similarly low-hanging fruit.) Then a proposed final solution to putting out the fire by literally excavating the whole thing was quoted at $660 million in 1984, which is $1.6 billion today (CPI).
I can't find estimates of the Centralia emissions but the Mulga, AL fire seems to put out a mean flux of 3,400 grams per square meter per day, and the Centralia fire covers 3,700 acres which is 15 million square meters, so if the flux is the same then 40 years of Centralia burning (note: it's actually expected to continue for centuries) creates 745 million metric tonnes of CO2. Thus, the gargantuan excavation project actually has a reasonably attractive effectiveness at $2.15 per metric ton CO2e averted, whereas a proactive early strategy would have a remarkable effectiveness of $0.007 per metric ton CO2e averted. For comparison, effective climate change charities are estimated to have impacts between $0.12 and $1 per metric tonne Co2e averted, the more widely quoted offset costs outside EA are mostly $3-10 per tonne, and the social cost of carbon (ignoring animal impacts) is usually estimated between $25 and $200 per tonne. With today's technology I imagine we might be able to do something much cheaper than excavating the entire burning area.
It's good to make sure that we have a causal story for why something would be inappropriately neglected, rather than just trusting numbers. In this case, extinguishing coal seam fires does not punish fossil fuel companies, which makes it less appealing to the public. In addition, the local victims are poor white American and foreign Chinese/Indian/Indonesian mining towns which (a) get comparatively little sympathy from the predominantly urban progressive Western environmentalist movement and (b) are not very environmentalist themselves. Finally, it doesn't involve cool-looking new technology construction or virtuous tree-planting, it involves no notion of progress, just boring and dirty maintenance.
In several of the articles on coal seam fires I have seen statements that a lack of money constrains efforts to extinguish them. However as far as I can tell there are no ready channels for donating money to this work. That gap would have to be overcome by philanthropic entrepreneurs and research funders. Also, we could ask our representatives to push a relevant bill (it seems like a good bipartisan measure that could be supported by both Republicans and Democrats).