The Chernigovsky Skete is a smaller monastic complex about 4 kilometres southeast of the Trinity Lavra, belonging to the same monastery but operating with greater seclusion. It is notable for two things: the 1886 neo-Byzantine church ensemble designed by Nikolai Sultanov, whose five domes and decorative brickwork make it handsome above ground, and the extensive network of underground caves and prayer cells beneath it, which you can visit with a guide.
The underground section is the stranger attraction — a series of low-ceilinged tunnels with small prayer chapels cut into the walls, lit by candles, where hermit monks have prayed since the 1840s. The tour takes about 30 minutes and requires a headscarf for women; a torch is helpful. The skete can be reached on foot from Sergiev Posad (allow 45 minutes to an hour) or by taxi. Opening hours for the underground tour vary — check locally before making the trip specifically for it.