The Brussels Bourse (Stock Exchange) stands on the grounds of a Franciscan convent, Les Récollets, which succumbed over the centuries to wars, fire, and religious conflict. Excavations begun in 1988 uncovered the convent's foundations and a bunch of medieval tombs. There's now a small underground museum, Bruxella 1238, rue de la Bourse (tel. 02/279-43-50), on the site. The most important tomb is that of Duke of Brabant Jean I, who died in 1294. You visit here only on guided tours that depart from the Musée de la Ville, Wednesday 10:15am (English), and 11:15am and 3pm (French). The tour costs 3€ ($4.80).
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.