Elon Musk wants to nuke Mars to transform the atmosphere and make it more suitable for human life. The SpaceX CEO made a new reference to his grand plan on Friday, as he joked with a TV producer on Twitter that the Korolev crater “needs a warmup.”

The crater was shared in an image from the European Space Agency on Thursday, taken by the Mars Express High-Resolution Stereo Camera. The 50-mile-wide site, filled with ice, is situated in the northern lowlands of Mars. The crater hosts a pile of water ice 1.12 miles wide all year, thanks to the cold trap phenomenon: the crater’s floor lies 1.24 miles down from the rim, meaning that as air passes over the top, it cools and drops to create a protective layer of cold air to house the deeper levels of the ice.

Musk, who joked about warming up this ice, has long advocated for tapping up Mars’ frozen stores of carbon dioxide near the planet’s poles. The plan would mean humans would only require a small breathing apparatus to move around the surface. It’s unclear how humans could release these carbon stores, but Musk has a couple of ideas: either build artificial pulsating suns over either pole to accelerate climate change, or go for the nuclear option — quite literally — and detonate some warheads.

The Korolev crater. ESA/DLR/FU Berlin

See more: SpaceX’s Elon Musk Defends Terraforming Mars After Study Says It Won’t Work

Unfortunately, it’s not clear that either plan could work. Beyond the logistics of building a giant sun, a study published in July claimed that Mars’ total carbon dioxide stores amount to around 15 millibars of pressure, far lower than the 1,000 millibars found at the Earth’s sea level. Musk dismissed the study by claiming that “there’s a massive amount of CO2 on Mars adsorbed into soil that’d be released upon heating,” adding: “with enough energy via artificial or natural (sun) fusion, you can terraform almost any large, rocky body.”

Musk’s idea may be put to the test sooner rather than later, as SpaceX prepares for a manned mission to Mars as early as 2024. Musk claims that the firm could set up a colony on Mars within the next seven to 10 years.

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