Delve Greedily and Deep

Pops partway through any journey to a planet's core, when you cross into the mantle layer. Take a Drill Mod for the harder deep terrain (Barrier Buster pops on the way), plenty of tethers or a Portable Oxygenator, and just keep digging down - gravity fades as you descend, and this unlocks en route to Encounter With the Infinite.