Small underground nuclear bombs were used to put out well fires. https://www.amusingplanet.com/2018/09/how-soviets-put-out-oi... https://www.youtube.com/watch?v=S57Xq03njsc

A few ten megaton bomb would create a very small earthquake, quite probably only detectable by instruments. You'd need to wade into millions-ten of millions of megatons to create an extinction event.