As a quintessential quantum matter, realizing robust Wigner crystals and achieving their angstrom-scale imaging remain formidable challenges. Here, we devise an original charge-transfer-crystallization strategy to realize an intrinsic electron solid made of $4f$ electrons. The highly localized and correlated nature of $4f$ electrons gives rise to a heavy Wigner crystal. This unprecedented heaviness manifests as an electron solid with record-high density ($2.02\times {10}^{13}/{\mathrm{cm}}^{-2}$) and melting temperature ($>60\mathrm{K}$). $Q$-plus atomic force microscopy and Kelvin probe force microscopy are employed to directly image the Wigner crystal, by which the sub-unit-cell localization and electrostatic influence of Wigner electrons are accurately determined. Moreover, the collective tunneling is captured, underlining the quantum nature of a Wigner crystal. Our discovery presents the $4f$ system as a tunable platform with the charge-transfer degree of freedom to study the 2D many-body correlation of heavy electrons.

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• Charge order
• Flat bands
• Quantum phase transitions
• Wigner crystal
• Atomic force microscopy
• Liquid helium cooling
• Molecular beam epitaxy
• Scanning tunneling microscopy