Strange Unidentified Garden Objects in my raised tomato bed - please help identify.

This is UGO 1 and is about the size of a thick slice of medium tomato. https://imgur.com/FHUfwsn

This is UGO 1 again, but split open - yellow and black on the inside. https://imgur.com/cdXVf62

There's three of them that just showed up very quickly out of nowhere. There in the alfalfa mulch and seem to be hugging the 1/4" drip lines