Walking past the granite façade of King’s Chapel on Tremont Street, it’s easy to miss the narrow, dark path that snakes around the building. The path leads to a large iron gate, and behind that gate lies one of Boston’s most fascinating historical artifacts—a crypt containing, among others, a French knight and a poisoned adulteress.

Somewhere between 100 and 150 people are buried beneath the floorboards of the historic chapel, the first Anglican church in New England when it was consecrated in 1689. The present-day chapel was completed in 1754, and it was the place of worship for British Loyalists during the American Revolution. When King’s Chapel reopened after the war, in 1785, it was the country’s first Unitarian church and today boasts an active congregation. Throughout its history, many members of the congregation chose to be buried on site in the chapel’s crypt.

Bostonia recently took a tour of the crypt, led by King’s Chapel tour guides Theresa Cooney (GRS’15) and Lucas Griswold (CAS’11). This Halloween weekend, King’s Chapel, one of 16 stops along the Freedom Trail, will offer special nighttime tours inside one of Boston’s eeriest and oldest sites.

“The building is ambient, the electricity flickers, there’s always rumbling from a train,” says Cooney, a PhD candidate in the Graduate Division of Religious Studies. “There are a lot of people buried here who have had crazy lives. And while I haven’t had a ghost experience, would I want to hang out in the crypt alone at midnight? No way.”

Only 3 of the 21 tombs have monuments with the families’ names inscribed on them.

It’s hard to imagine the space below as you enter the 260-year-old building from the street level. The colonial-revival-style church has a soaring ceiling, wood carvings, and a large brass chandelier. Tour visitors can climb the rickety original 18th-century steps to the chapel’s bell tower. The original bell became cracked in 1814, and two years later Paul Revere cast its replacement. (It was the last bell the silversmith and Revolutionary war patriot ever cast.) The 2,437-pound bell is still rung today, using a long rope. Tours of the chapel do not include a visit to King’s Chapel Burying Ground next door—dating back to 1636, it is the oldest burial ground in the city. “Burying ground actually means a nonreligious burial,” says Griswold, who studied archaeology at BU. The burying ground can be visited separately at no cost.

As you enter the crypt, the first things you notice are the shadows and the bricked, barrel-vaulted tombs. The 21 tombs here were installed in 1760, although spotty record-keeping means no one knows for sure who the first occupant was. “What we do know is that most of them were privately owned by wealthy members of the congregation,” Cooney says, like the resident in tomb number two—Frances “Franny” Apthorp, who bore a child by her sister’s husband, and poisoned herself shortly after.

Some remains can be found in the back of the crypt in what is called the Strangers’ Tomb; it’s estimated that between 30 and 50 souls are spending their eternal lives here. The use of the word “stranger” doesn’t mean unknown, notes Cooney. Rather “it could mean stranger to each other, or stranger to the church. Sometimes the congregation would take the poor under their wing and then bury them here.”

When the crypt was in use, coffins would be carried from the church around to the side of the building, and then the crypt’s iron gate would be opened, says Theresa Cooney (GRS’15).

One of the Strangers’ Tomb’s stranger occupants is a man by the name of Chevalier de Saint Sauveur. A French knight and naval officer, he arrived in the Colonies in 1778 on his ship Le Tonnant. In the midst of a series of riots during the American Revolution, people looking for food mobbed the ship. He was struck over the eye with a chair leg and died from his injuries eight days later.

George Washington, commander-in-chief of the Continental Army, didn’t want to lose his new French allies and needed to find a place to bury the Catholic French naval officer. Since Catholicism was banned in the country at the time, a secret Catholic service was held in the chapel’s crypt for the chevalier, and he was then buried inside the Strangers’ Tomb. An obelisk monument dedicated to de Saint Sauveur stands in the front of King’s Chapel.

Around the corner from the Strangers’ Tomb is the tomb belonging to the Coolidge family, distant relatives of the country’s 30th president, Calvin Coolidge. A brick has been removed from the crypt’s stone wall to check for structural damage, so it’s possible to peer inside the vault during the tour. Several coffins stacked on top of one another can be seen. One is tiny; it is believed to contain the remains of an infant. Another is without a lid (“Coffins sometimes erupt,” Griswold says, because of atmospheric conditions) and a powdered wig is visible poking out.

Coffins in early America were simple, Cooney says, made out of wood and oftentimes with an octagonal shape. Embalming wasn’t common until the Civil War, according to Griswold, so balsam and sawdust were routinely placed in coffins to make them less “noxious.”

Occasionally, a tomb has to be opened to check for structural damage in the church’s foundation. When this happens, it’s possible to peek inside and see who lies within.

One of the last stops in the crypt tour is the Bulfinch tomb, although no one by the name of Bulfinch resides there today. Charles Bulfinch, one of America’s first architects and designer of the Massachusetts State House and much of the US Capitol, was laid to rest here with his wife following his death in 1844. Their son decided to reinter them at the newly built Mount Auburn Cemetery, in Cambridge, Mass., in 1850.

The “rural cemetery movement” that began in America in the 19th century had two major purposes, Griswold says. The first was that it eliminated the problem of over-burial and therefore disease, which commonly plagued cities. Second, cemeteries like Mount Auburn were designed to be beautiful places, with preplanned paths, water features, and trees, where families could visit a “sleeping” loved one. In the 1890s, city of Boston ordinances banned the mingling of the living and the dead, Griswold says. That occurred four decades after the last burial took place in the King’s Chapel crypt.

“This place has amazing energy,” Cooney says. “There’s over 300 years of history down here.”

For tourists, King’s Chapel is open Monday through Saturday, 10 a.m. to 5 p.m., and 1:30 to 5 p.m. Sunday (hours change between Memorial Day and Labor Day). Special nighttime Halloween tours run from 4 to 7 p.m. and can be arranged 72 hours in advance by calling 617-523-1749. Admission is $5 to tour either the crypt or the bell tower; $8 to tour both. The church is at 58 Tremont St., Boston (take the MBTA Green Line to Park Street). The worship schedule is here. More information is here.