Many of today's software systems will become the legacy systems of tomorrow, comprised of outdated technology and inaccurate design documents. Preparing for their eventual re-engineering requires engineers to regain lost design knowledge and discover re-engineering opportunities. While tools and visualizations exist, comprehending an unfamiliar code base remains challenging. Hence, software archaeology suffers from a considerable entry barrier as it requires expert knowledge, significant diligence, tenacity, and stamina. In this paper, we propose a paradigm shift in how legacy systems' design knowledge can be regained by presenting our vision for an immersive explorable software visualization in virtual reality (VR). We propose innovative concepts leveraging benefits of VR for a) immersion in an exoteric visualization metaphor, b) effective navigation and orientation, c) guiding exploration, and d) maintaining a link to the implementation. By enabling immersive and playful legacy system exploration, we strive for lowering the entry barrier, fostering long-term engagement, strengthening mental-model building, and improving knowledge retention in an effort to ease coping with the increased number of tomorrow's legacy systems.