Wow it's a fantastic journey, maybe it's needed more than being immortal for one week to accomplish and diving in lava hehe, but I would try to enter through the poles of the earth ,where is being said there are holes that address to the inner earth and to a city called Agartha. Altought maybe it would take months or years to find the entrance.
Anyways your goals and the way to make it's clear for you, it would be pretty interesting to see the travel to the center of earth, greetings.
The easy option would be to build a subterranean craft that took me down to the centre of the earth, but I don't always take the easy option, so swimming down there is the way to go, it makes one appreciate being there after exerting that effort to get there.