It depends on what "secure" means. Any lock can be destroyed with tools. Most locks can be broken with a big pair of bolt cutters, a drill, or, failing that, melting.
If secure means "without leaving evidence of tampering," things get a lot more interesting, but that has narrow practical use cases outside of stuff like espionage. Once you're in this space, we can start talking about how difficult something can be without specialized tools. But now we're leaving "I am protecting my stuff" territory and entering "this is just a sport and we're agreeing on a ruleset" territory.
There are a couple of lock designs out there that I don't think anybody's successfully ever picked. The ones that first come to mind are a couple of the "smart" electronic locks. Many of those are junk, but a few are very well thought out.
If secure means "without leaving evidence of tampering," things get a lot more interesting, but that has narrow practical use cases outside of stuff like espionage. Once you're in this space, we can start talking about how difficult something can be without specialized tools. But now we're leaving "I am protecting my stuff" territory and entering "this is just a sport and we're agreeing on a ruleset" territory.
There are a couple of lock designs out there that I don't think anybody's successfully ever picked. The ones that first come to mind are a couple of the "smart" electronic locks. Many of those are junk, but a few are very well thought out.