You've ridden the electron shells, the gap, the dead center. This is the thing at 0,0,0 — the actual core. A speck ten-thousand times smaller than its atom, holding essentially all the mass, run by the strongest force in nature, and itself made of smaller things held by an even stranger one. Four questions, four panels.
The nucleus is the tiny, dense, positively-charged knot at the center of every atom, made of protons and neutrons packed together. Almost nothing about it is intuitive from its size. If the whole atom were a sports stadium, the nucleus would be a grain of rice at the center spot — verified ratio ~33,000× — and the electrons would be a faint haze out in the stands. Yet that grain holds over 99.9% of the atom's mass. Its density is the part that breaks the mind: about 2.3×10¹⁷ kg/m³, meaning a single teaspoon of pure nuclear matter would weigh ~a billion tonnes — a mountain in a spoon. A neutron star is, essentially, a nucleus the size of a city.
The nucleus has a paradox to solve: protons are all positively charged, and like charges repel — fiercely, at these tiny distances. A nucleus of many protons should blow itself apart instantly by electromagnetic repulsion. It doesn't, because of the strong nuclear force — about 137× stronger than electromagnetism at contact range, but with a catch: it only reaches ~1–3 femtometers, barely past touching. So the nucleus is a constant tug-of-war: strong force gluing neighbors together (short reach, very strong) vs. electromagnetic repulsion pushing all protons apart (long reach, weaker but relentless). Neutrons are the peacekeepers — they feel the strong glue but carry no charge, so they add binding without adding repulsion. That's why big nuclei need extra neutrons to survive, and why past a point (~92 protons, uranium) nothing is truly stable: the repulsion finally out-reaches the glue.
Zoom in and the nucleus isn't fundamental. Each proton and neutron is itself made of three quarks bound by gluons — proton = up-up-down, neutron = up-down-down — held by the strong force in its original form (the nuclear force between protons and neutrons is just the leftover spillover of this deeper binding). The mind-bending part, verified earlier this session: the quarks' own mass is almost nothing. ~99% of a proton's mass is the energy of the gluon field and the quarks' motion — confined energy registering as mass via E=mc². You are made of atoms; atoms are mostly empty; the tiny full part is mostly not matter but bound energy wearing mass. The floor, as far as we can currently see: quarks and gluons, with no known substructure. That's where the turtles stop — for now.