there is a mod called tropicraft(i don't know if it is still being updated though), that doesn't only add volcanoes with a large amount of lava, but they even spawn fireballs of death spawning lava where they hit(once they are full, before they are, they fill up).
Volcanoes with Surface-Accessible Central Lava Chambers™
Lava does not exist in "chambers", because it is by definition exposed to the surface (although lava lakes found in vanilla Minecraft would satisfy these conditions). Magma chambers are a real thing, but by definition they are not exposed to the surface; in fact, magma chambers are at least 1km underground.
drain below Bedrock (which is presumably supposed to be the mantle)
Since magma chambers are farther underground than 64m (the accessable world height in Minecraft) this sort of thing would be (quite) below the Bedrock layer. I think the traditional mindset is that the underside of the Bedrock layer is the Nether, however. Maybe that's why there aren't many volcano mods?
Now what would be cool is a reworking of Geothermal Generators and Pumps to create renewable power if the Pump reaches down (with Mining Pipes) to Bedrock within the vicinity of a Volcano. Of course, there would have to be a maximum amount of energy per chunk which could be safely extracted without causing local depletion of resources; exceeding this threshold risks diminishing returns, and there is always the risk of a volcanic explosion which causes eg: Pahoehoe Lava flows (fire grief), a nice big particle effect, ash which piles up like snow and sufficates entities, etc. Maybe a Geothermal Generator multiblock using Liquid Heat Exchangers?
