In the mid-80's I was working in Indonesia and managed to visit 20+ volcanoes, mostly active. All were very atmospheric. The one that stands out most was Keli Mutu on Flores. It's famous for 3 crater lakes all of different colours. When I was there (1985) one was dark red, one was light-blue and the other was green-blue. The craters are spherical, about 500m across and 300m from rim to top of the lakes. The sides of the craters of vertical.
While I was walking around them I decided to chuck some rocks in. Local tradition has it that the lakes are inhabited by various spirits (souls of virgins, warriors etc...) and I thought it would be fun to try and stir them up. It must be boring inhabiting crater lakes for years on end. I took a hand sized rock and lobbed it in. I lost sight of the rock on it's way down and saw no splash, so I decided to throw something bigger. I then spent the next hour throwing rocks of all sizes into the lakes without any success. In every case I lost sight of the rock and didn't hear or see any splash. Eventually I got bored and left. I figured that the walls of the craters weren't actually vertical and that it was only an optical illusion that they were. As I was leaving a large slab of the rim, about the size of a house, collapsed into the lake 50m from where I was standing. As I couldn't see into the lake I didn't see it fall into the lake, but there was no sound at all.
A couple of days later I met a bloke in a cafe in Ende, the main town on Flores. He asked me if I'd visited Keli Mutu. I said I had. He asked me if I'd try to throw any rocks into the lakes. Again I said I had. He asked me if any had landed in any lakes, I told him no and it was strange because the walls of the craters were vertical and I was standing right on the edge. He said due to a strong magnetic field around the craters it was impossible to to hit the lakes.
In the mid-90's I was working as a geologist for an exploration company in Indonesia. One of our contracts covered Flores. We carried out an aeromagnetic survey of most of the island. Unfortunately Keli Mutu was just outside our contract area and wasn't fully covered by the survey. The portions of the survey that did cover the outer slopes of the Keli Mutu region showed nothing unusual for volcanic areas in that part of the world. I wasn't involved in field work on the Flores contract, so I haven't had a chance to revisit Keli Mutu, but I can tell you next time I visit I'll spend as long as it takes to throw something in, even if I have to climb down the crater wall.
Separately, about driiling into the Moho. I thought it was a joint Russia-Finland project drilling somewhere near their border. The area has thin crust due to the weight of ice during the last ice age that is still slowly rebounding. In 1987, I think I read, they were down to about 10km drilling through a sort of high grade metamorphosed granite called eclogite, and they were having lots of technical problems.
Cheers