It’s more fun to do it with nuclear weapons.
While powering it with nukes is silly sci-fi stuff, the idea of using a lava pit for power is actually being done (kinda). There are some solar power plants that basically use a bunch of mirrors to focus the sun’s rays on a pit filled with salt. The salt gets so hot that it melts, and the heat from the molten salt is used to turn water into steam, which is then used to power a conventional steam turbine type generator.
This type of solar plant doesn’t require expensive photovoltaic panels and also continues to provide power at night since the molten salt contains so much heat that it won’t solidify and remains molten even through the dark hours.
A couple of days with little to no sun due to cloud cover would allow the molten salt to cool down and solidify, so these types of solar plants only work in sunny areas like the Southwestern US.
And it’s *still *fusion powered. Rule of Cool yet applies!
We fly over this molten salt project fairly often Crescent Dunes Solar Energy Project - Wikipedia
Damn those mirrors are bright. Sometimes we get a little too much concentrated fusion power on us and we’re 20 miles way from the darn thing.
Lava isn’t exactly a homogeneous substance … it’s density can change and I’m wondering if having the lava overflow on a regular basis might not stop this particular show … we have these environments scattered around the globe … the problem is some of them have a tendency to explode at a Biblical magnitude … big enough to put any Mark 39 to shame … rock in it’s liquid state is tricky stuff to deal with …
This is awesome, I had never heard of the project before.
I like how it gives many technical details, and then the way it’s written it looks like they are providing a citation to support the claim that sunlight is unavailable at night.
Well, using lava for power is actually being done way more than kinda. Heck I’ve seen it done commercially. If you can get to Iceland, you can see it too
Though it is ultimately only fission-derived energy, so maybe not quite as cool as fusion power.
Here’s some information on a US attempt at the OP’s idea from back in the 1960s.
3.1kt underground detonation. 6 months later it was still about 140F in the cavity.
File this all under “Ideas, Questionable”…
Here’s one 4x as big. Ivanpah Solar Power Facility - Wikipedia
These things have been sprouting like weeds out there where there’s few if any clouds.
I’ll bet with a little extra software development on the mirror aiming system, we could turn that into a 2020’s-style death ray.
As an aside, the OP appears to have been the first, and so far only, person to ever use the very apt word “hellpit” to describe this type of design, or at least so suggests Google. So, kudos on that.
Alas, I plagiarized the name from DC Comics fictional planet Apokolips.
In your honor earlier tonight I referred to Venus as a planetary hellpit of boiling sulfuric acid. Seemed apt.
It was a very pretty hellpit hanging high in the western sky at dusk. But a hellpit nonetheless.
It’s already tested and operational:
“Living next to a hellpit has its disadvantages, but you should see when the birds fly over it…”
To be clear, I didn’t say you were the only one to use the word “hellpit”. I said that you were the only one to use it for power generation via underground nuclear explosions.
So are solar panels, wind turbines, hydroelectric, fossil fuels, and all other methods that rely on the current or past output of the sun. (Geothermal and fission rely on the fusion output of earlier generations of stars.)
Spoilsport! 
One problem might be the radiated thermal losses from the exposed hot surface of the hellpit. I remember this thread, which discussed the challenges of maintaining a lava-filled moat. One of the respondents calculated an ongoing thermal radiative loss of about 24 kW per square meter of exposed surface. A hellpit comprising a “large lake or small sea” will thus have massive thermal losses; imagine a surface 250 meters square (= 62,500 square meters), and your thermal losses will be a continuous 1.5 gigawatts. Thermal power plant efficiency is on the order of 50%, which means that if you’re generating 1.5 gigawatts of electrical power, you’re already going to have 1.5 gigawatts of waste heat to get rid of; add in the 1.5 gigawatts of thermal loss from the surface of the hellpit, and your overall efficiency drops to 33%. If you can build and deploy thermonuclear bombs fairly cheaply, then maybe the low overall efficiency isn’t a big deal, but I doubt that’s the case.
Looks like the pit will need to be roofed over; maybe just as well to keep radioactive material from boiling out. Speaking of which, it might be a good place to dump the world’s radioactive waste- it isn’t like it would make things any worse.