There are two Bronze Age entrance graves at Innisidgen Hill on St Mary's. This type of grave is unique to Scilly and West Cornwall in Britain, and is named after the long, open passage that led to a roofed chamber. The passage and chamber are contained inside a round-ish mound. The pair at Innisidgen are definitely worth the walk (about two and a half miles from Hugh Town), not least for the beautiful views.
The Upper Innisidgen entrance grave (also known as Innisidgen Carn, Innisidgen Upper Burial Chamber and the Giant's Grave) is one of the best preserved on Scilly. It's also pretty big, measuring 30' by 26', and is about 6' high. The five capstones that make the roof of the chamber are huge. It's thought that its stone passageway would have been around 15' long, making it a good example of a larger entrance grave.
The Lower burial chamber hasn't survived as well as its peer. Only two of its capstones remain, and it's harder to work out what's what when you look at it. Beyond the graves, there's the remains of a prehistoric field system. Establishing burial sites on agricultural land seems to have been a regular practice on Scilly, and you'll see a similar juxtaposition at Porth Hellick.
The monument is managed by English Heritage, and it's free to visit (it's open during daylight hours). The best (realistically, only) way to get here is on foot, and it's a bit of a hilly and bumpy walk.