Ditch the sandals; the floor is slick, wet, and freezing even in mid-July. Spend 20 minutes crouching under sharp, lava-formed ceilings that get uncomfortably low, so watch your head. Bring a legit jacket because the temp stays near 0°C inside. Skip the unreliable local buses and book a private car tour from Tokyo to handle the transit logistics. Wear shoes you don't mind ruining, and prepare for a genuinely dark, claustrophobic subterranean vibe.
